Eplain ¶

4.1 Diagnostics ¶

Plain TeX provides the \tracingall command, to turn on the maximum amount of tracing possible in TeX. The (usually voluminous) output from \tracingall goes both on the terminal and into the transcript file. It is sometimes easier to have the output go only to the transcript file, so you can peruse it at your leisure and not obscure other output to the terminal. So, Eplain provides the command \loggingall. (For some reason, this command is available in Metafont, but not in TeX.)

It is also sometimes useful to see the complete contents of boxes. \tracingboxes does this. (It doesn’t affect whether or not the contents are shown on the terminal.)

You can turn off all tracing with \tracingoff.

You can also turn logging on and off globally, so you don’t have to worry about whether or not you’re inside a group at the time of command. These variants are named \gloggingall and \gtracingall.

Finally, if you write your own help messages (see \newhelp in The TeXbook), you want a convenient way to break lines in them. This is what TeX’s \newlinechar parameter is for; however, plain TeX doesn’t set \newlinechar. Therefore, Eplain defines it to be the character ^^J.

For example, one of Eplain’s own error messages is defined as follows:

\newhelp\envhelp{Perhaps you forgot to end the previous^^J%
   environment? I'm finishing off the current group,^^J%
   hoping that will fix it.}%

4.2 Rules ¶

The default dimensions of rules are defined in chapter 21 of the The TeXbook. To sum up what is given there, the “thickness” of rules is 0.4pt by default. Eplain defines three parameters that let you change this dimension: \hruledefaultheight, \hruledefaultdepth, and \vruledefaultwidth. By default, they are defined as The TeXbook describes.

But it would be wrong to redefine \hrule and \vrule. For one thing, some macros in plain TeX depend on the default dimensions being used; for another, rules are used quite heavily, and the performance impact of making it a macro can be noticeable. Therefore, to take advantage of the default rule parameters, you must use \ehrule and \evrule.

4.3 Citations and bibliographies ¶

Bibliographies are part of almost every technical document. To handle them conveniently, you need two things: a program to do the tedious formatting, and a way to cite references by labels, rather than by numbers. The BibTeX program, written by Oren Patashnik, takes care of the first item; the citation commands in LaTeX, written to be used with BibTeX, take care of the second. Therefore, Eplain adopts the use of BibTeX, and virtually the same interface as LaTeX.

The general idea is that you put citation commands in the text of your document, and commands saying where the bibliography data is. When you run TeX, these commands produce output on the file with the same root name as your document (by default) and the extension .aux. BibTeX reads this file. You should put the bibliography data in a file or files with the extension .bib. BibTeX writes out a file with the same root name as your document and extension .bbl. Eplain reads this file the next time you run your document through TeX. (It takes multiple passes to get everything straight, because usually after seeing your bibliography typeset, you want to make changes in the .bib file, which means you have to run BibTeX again, which means you have to run TeX again…) An annotated example of the whole process is given below.

If your document has more than one bibliography—for example, if it is a collection of papers—you can tell Eplain to use a different root name for the .bbl file by defining the control sequence \bblfilebasename. The default definition is simply \jobname.

On the other hand, if your document’s bibliography is very simple, you may prefer to create the .bbl file yourself, by hand, instead of using BibTeX. An annotated example of this approach is also given below.

See the document BibTeXing (whose text is in the file btxdoc.tex, which should be in the Eplain distribution you got) for information on how to write your .bib files. Both the BibTeX and the Eplain distributions contain several examples, also.

The \cite command produces a citation in the text of your document. The exact printed form the citation will take is under your control (see Formatting citations). \cite takes one required argument, a comma-separated list of cross-reference labels (see Cross-references, for exactly what characters are allowed in such labels). Warning: spaces in this list are taken as part of the following label name, which is probably not what you expect. The \cite command also produces a command in the .aux file that tells BibTeX to retrieve the given reference(s) from the .bib file. \cite also takes one optional argument, which you specify within square brackets, as in LaTeX. This text is simply typeset after the citations. (See the example below.)

Eplain can create hypertext links for citations pointing to the relevant bibliography entries (see Citation hyperlinks: cite, bib).

Another command, \nocite, puts the given reference(s) into the bibliography, but produces nothing in the text.

The \bibliography command is next. It serves two purposes: producing the typeset bibliography, and telling BibTeX the root names of the .bib files. Therefore, the argument to \bibliography is a comma separated list of the .bib files (without the ‘.bib’). Again, spaces in this list are significant.

You tell BibTeX the particular style in which you want your bibliography typeset with one more command: \bibliographystyle. The argument to this is a single filename style, which tells BibTeX to look for a file style.bst.

Numerous styles have been defined by now; see https://www.ctan.org/topic/bibtex-sty. One particular case: the apalike semi-standard style requires \input apalike.tex to function properly (else text is overwritten); other APA-like (“humanities”) styles surely also require this or similar changes.

See the document Designing BibTeX styles (whose text is in the btxhak.tex) for information on how to write your own styles.

Eplain automatically reads the citations from the .aux file when your job starts.

If you don’t want to see the messages about undefined citations, you can say \xrefwarningfalse before making any citations. Eplain automatically does this if the .aux file does not exist. You can restore the default by saying \xrefwarningtrue.

Here is a TeX input file that illustrates the various commands.

\input eplain                    % Reads the .aux file.
Two citations to Knuthian works:
  \cite[note]{surreal,concrete-math}.
\beginsection{References.}\par   % Title for the bibliography.
\bibliography{knuth}             % Use knuth.bib for the labels.
\bibliographystyle{plain}        % Number the references.
\end                             % End of the document.

If we suppose that this file was named citex.tex and that the bibliography data is in knuth.bib (as the above \bibliography command says), the following commands do what’s required. (‘$ ’ represents the shell prompt.)

$ tex citex     (produces undefined citation messages)
$ bibtex citex  (read knuth.bib and citex.aux, write citex.bbl)
$ tex citex     (read citex.bbl, still have undefined citations)
$ tex citex     (one more time, to resolve the references)

The texi2dvi program can help you automate this process (see Invoking Eplain).

For simple documents you might choose to write the .bbl file yourself, instead of running BibTeX. For this scenario, the following commands should suffice:

$ tex citex     (read citex.bbl, produces undefined citation messages)
$ tex citex     (one more time, to resolve the references)

The output looks something like (because we used the plain bibliography style):

Two citations to Knuthian works: [2,1 note].

References

[1] Ronald L. Graham, Donald E. Knuth, and Oren Patashnik. Concrete Mathematics. Addison-Wesley, Reading, Massachusetts, 1989.

[2] Donald E. Knuth. Surreal Numbers. Addison-Wesley, Reading, Massachusetts, 1974.

See the BibTeX documentation for information on how to write the bibliography databases, and the bibliography styles that are available. (If you want your references printed with names, as in [Knu74], instead of numbered, the bibliography style is alpha.)

• Formatting citations
• Formatting bibliographies
• Commands from LaTeX

\def\printcitestart{\unskip $^\bgroup}
\def\printbetweencitations{,}
\def\printcitefinish{\egroup$}
\def\printcitenote#1{\hbox{\sevenrm\space (#1)}}

4.4 Displays ¶

By default, TeX centers displayed material. (Displayed material is just whatever you put between $$’s—it’s not necessarily mathematics.) Many layouts would be better served if the displayed material was left-justified. Therefore, Eplain provides the command \leftdisplays, which indents displayed material by \parindent plus \leftskip, plus \leftdisplayindent.

You can go back to centering displays with \centereddisplays. (It is usually poor typography to have both centered and left-justified displays in a single publication, though.)

\leftdisplays also changes the plain TeX commands that deal with alignments inside math displays, \displaylines, \eqalignno, and \leqalignno, to produce left-justified text. You can still override this formatting by inserting \hfill glue, as explained in The TeXbook.

Eplain defines \eqnum and \eqalignnum which can be set up to produce either left-aligned or right-aligned equation numbers. \lefteqnumbers (\righteqnumbers) will define \eqnum to expand to \eqno (\leqno), and \eqalignnum to expand to \eqalignno (\leqalignno). Default is \righteqnumbers (right-aligned equation numbers).

• Formatting displays

4.5 Time of day ¶

TeX provides the day, month, and year as numeric quantities (unless your TeX implementation is woefully deficient). Eplain provides some control sequences to make them a little more friendly to humans.

\monthname produces the name of the current month, abbreviated to three letters.

\fullmonthname produces the name of the current month, unabbreviated (in English).

\timestring produces the current time, as in ‘1:14 p.m.’

\timestamp produces the current date and time, as in ‘23 Apr 64 1:14 p.m.’. (Except the spacing is slightly different.)

\today produces the current date, as in ‘23 April 1964’.

4.6 Lists ¶

Many documents require lists of items, either numbered or simply enumerated. Plain TeX defines one macro to help with creating lists, \item, but that is insufficient in many cases. Therefore, Eplain provides two pairs of commands:

\numberedlist … \endnumberedlist ¶

\orderedlist … \endorderedlist

These commands (they are synonyms) produce a list with the items numbered sequentially, starting from one. A nested \numberedlist labels the items with lowercase letters, starting with ‘a’. Another nested \numberedlist labels the items with roman numerals. Yet more deeply nested numbered lists label items with ‘*’.

\unorderedlist … \endunorderedlist ¶

This produces a list with the items labelled with small black boxes (“square bullets”). A nested \unorderedlist labels items with em-dashes. Doubly (and deeper) nested unordered lists label items with ‘*’s.

The two kinds of lists can be nested within each other, as well.

In both kinds of lists, you begin an item with \li. An item may continue for several paragraphs. Each item starts a paragraph.

You can give \li an optional argument, a cross-reference label. It’s defined to be the “marker” for the current item. This is useful if the list items are numbered. You can produce the value of the label with \xrefn. See Cross-references.

Eplain can create hypertext links for the markers produced by \xrefn pointing to the relevant list item (see List hyperlinks: li).

You can also say \listcompact right after \numberedlist or \unorderedlist. The items in the list will then not have any extra space between them (see Formatting lists). You might want to do this if the items in this particular list are short.

Here is an example:

\numberedlist\listcompact
\li The first item.
\li The second item.

The second paragraph of the second item.
\endnumberedlist

• Formatting lists

4.7 Verbatim listing ¶

It is sometimes useful to include a file verbatim in your document; for example, part of a computer program. The \listing command is given one argument, a filename, and produces the contents of that file in your document. \listing expands \listingfont to set the current font. The default value of \listingfont is \tt.

You can take arbitrary actions before reading the file by defining the macro \setuplistinghook. This is expanded just before the file is input.

If you want to have line numbers on the output, you can say \let\setuplistinghook = \linenumberedlisting. The line numbers are stored in the count register \lineno while the file is being read. You can redefine the macro \printlistinglineno to change how they are printed.

Normally, the \listing command will add a final empty line at the end of the output, even if the file does not end in a newline. To suppress this final line, you can say \let\setuplistinghook = \nolastlinelisting. This also works with line numbers (say \def\setuplistinghook{\linenumberedlisting \nolastlinelisting}), but only if \printlistinglineno consists exclusively of boxes at the top level (i.e., any \kerns or glue should be wrapped up in a box).

You can use the form feed control character (ASCII code 12, typed as CTRL-L) in the file to force a page break in the output.

You can produce in-line verbatim text in your document with \verbatim. End the text with |endverbatim. If you need a ‘|’ in the text, double it. If the first character of the verbatim text is a space, use | . (| will work elsewhere in the argument, too, but isn’t necessary.)

For example:

\verbatim| ||\#%&!|endverbatim

produces |\#%&!.

Line breaks and spaces in the verbatim text are preserved.

You can change the verbatim escape character from the default ‘|’ with \verbatimescapechar char; for example, this changes it to ‘@’.

\verbatimescapechar \@

The backslash is not necessary in some cases, but is in others, depending on the catcode of the character. The argument to \verbatimescapechar is used as \catcode `char, so the exact rules follow that for \catcode.

To reset the category code of all special characters to 12 (“other”), \verbatim uses \uncatcodespecials (see Category codes). If you make additional characters “special”, you should extend \dospecials to include those characters, lest they be given special treatment inside verbatim environments. For example,

\catcode`\A=\active
% Try commenting out the following line.
\expandafter\def\expandafter\dospecials\expandafter{\dospecials\do\A}
\verbatimA#$%_^|endverbatim

Because \verbatim must change the category code of special characters, calling inside a macro definition of your own does not work properly. For example:

\def\mymacro{\verbatim &#%|endverbatim}% Doesn't work!

To accomplish this, you must change the category codes yourself before making the macro definition. Perhaps \uncatcodespecials will help you (see Category codes).

4.8 Contents ¶

Producing a table of contents that is both useful and aesthetic is one of the most difficult design problems in any work. Naturally, Eplain does not pretend to solve the design problem. Collecting the raw data for a table of contents, however, is much the same across documents. Eplain uses an auxiliary file with extension .toc (and the same root name as your document) to save the information.

• Writing the .toc file
• Reading the .toc file
• Changing the .toc file’s root name
• Alternative contents files

\writenumberedtocentry{chapter}{A $\sin$ wave}{\the\chapno}
\writetocentry{section}{A section title}

\tocchapterentry{A $\sin$ wave}{3}{8}
\tocsectionentry{A section title}{9}

\writenumberedtocline{chapter}{\the\chapno}{A $\sin$ wave}

\tocchapterentry{3}{A $\sin$ wave}{8}

\writenumberedtocentry{1}{A $\sin$ wave}{\the\chapno}
\writenumberedtocline{1}{\the\chapno}{A $\sin$ wave}
\writetocentry{2}{A section title}

\tocentry{1}{A $\sin$ wave}{3}{8}
\tocentry{1}{3}{A $\sin$ wave}{8}
\tocentry{2}{A section title}{9}

\definecontentsfile{toc}

4.9 Cross-references ¶

It is often useful to refer the reader to other parts of your document; but putting literal page, section, equation, or whatever numbers in the text is certainly a bad thing.

Eplain therefore provides commands for symbolic cross-references. It uses an auxiliary file with extension .aux (and the same root name as your document) to keep track of the information. Therefore, it takes two passes to get the cross-references right—one to write them out, and one to read them in. Eplain automatically reads the .aux file at the first reference; after reading it, Eplain reopens it for writing.

You can control whether or not Eplain warns you about undefined labels. See Citations and bibliographies.

Labels in Eplain’s cross-reference commands can use characters of category code eleven (letter), twelve (other), ten (space), three (math shift), four (alignment tab), seven (superscript), or eight (subscript). For example, ‘(a1 $&^_’ is a valid label (assuming the category codes of plain TeX), but ‘%#\{’ has no valid characters.

You can also do symbolic cross-references for bibliographic citations and list items. See Citations and bibliographies, and Lists.

Eplain can create hypertext links for the cross-references (see Cross-reference hyperlinks: definexref, ref).

• Defining generic references
• Using generic references

\definexref{sec-intro}{3.1}{section}

\def\sectionword{Section}
\definexref{sec-intro}{3.1}{section}
\definexref{sec-next}{3.2}{section}
See \refs{sec-intro} and \refn{sec-next} ...

4.10 Page references ¶

Eplain provides two commands for handling references to page numbers, one for definition and one for use.

\xrdef{label} ¶

Define label to be the current page number. This produces no printed output, and ignores following spaces.

\xref{label} ¶

Produce the text ‘p. pageno’, which is the usual form for cross-references. The pageno is actually label’s definition; if label isn’t defined, the text of the label itself is printed. The ‘p. ’ prefix is defined by \xrefpageword. Its default definition is p.\thinspace.

Eplain can create hypertext links for the page references (see Page reference hyperlinks: xrdef, xref).

4.11 Equation references ¶

Instead of referring to pages, it’s most useful if equation labels refer to equation numbers. Therefore, Eplain reserves a \count register, \eqnumber, for the current equation number, and increments it at each numbered equation.

Here are the commands to define equation labels and then refer to them:

\eqdef{label} ¶

This defines label to be the current value of \eqnumber, and, if the current context is not inner, then produces a \eqnum command (see Displays). (The condition makes it possible to use \eqdef in an \eqalignno construction, for example.) The text of the equation number is produced using \eqprint. See Formatting equation references.

If label is empty, you still get an equation number (although naturally you can’t reliably refer to it). This is useful if you want to put numbers on all equations in your document, and you don’t want to think up unique labels.

To refer to the last equation with the empty label, you use the empty label in one of the equation reference macros (see below). This can be handy when you want to refer to an equation shortly after its definition, say, in the sentence following the displayed equation, and do not intend to refer to the equation later. But use this trick with extreme caution: if later you change the text and insert another empty definition between the original definition and the reference, the reference will start to refer to the new empty-labeled equation.

\eqdefn{label} ¶

This is like \eqdef, except it always omits the \eqnum command. It can therefore be used in places where \eqdef can’t; for example, in a non-displayed equation. The text of the equation number is not produced, so you can also use it in the (admittedly unusual) circumstance when you want to define an equation label but not print that label.

\eqref{label} ¶

This produces a formatted reference to label. If label is undefined (perhaps because it is a forward reference), it just produces the text of the label itself. Otherwise, it calls \eqprint.

\eqrefn{label} ¶

This produces the cross-reference text for label. That is, it is like \eqref, except it doesn’t call \eqprint.

Equation labels can contain the same characters that are valid in general cross-references.

Eplain can create hypertext links for the equation references (see Equation reference hyperlinks: eq).

• Formatting equation references
• Subequation references

\def\eqprint#1{{\it (#1)}}

\def\eqconstruct#1{\the\chapternumber.#1}

$$...\eqdef{a-eq}$$
...
$$...\eqdef[\eqrefn{a-eq}*]{a-eq-var}$$
In \eqref{a-eq-var}, we expand on \eqref{a-eq}, ...

\newcount\eqpartnum

\def\eqconstruct#1{%
  \global\eqpartnum=\the\partnum\relax
  \number\chapnum.#1%
}

\def\eqprint#1{%
  \setbox0=\hbox{#1}%
  (\ifnum\partnum=\eqpartnum \else
     \uppercase\expandafter{\romannumeral\eqpartnum}.%
   \fi
   \box0)%
}%

\def\eqsubreftext#1#2{#1.#2}%

\newcount\subref
\def\eqsubreftext#1#2{%
  \subref = #2           % The space stops a <number>.
  \advance\subref by 96  % `a' is character code 97.
  #1\char\subref
}

4.12 Indexing ¶

Eplain provides support for generating raw material for an index, and for typesetting a sorted index. A separate program must do the actual collection and sorting of terms, because TeX itself has no support for sorting.

Eplain can create hypertext links pointing from the index to the index terms (see Index hyperlinks: idx).

Eplain’s indexing commands were designed to work with the program MakeIndex (https://ctan.org/pkg/makeindex); MakeIndex is also commonly included in prepackaged TeX distributions. It is beyond the scope of this manual to explain how to run MakeIndex, and all of its many options.

The basic strategy for indexing works like this:

1. For a document foo.tex, Eplain’s indexing commands (e.g., \idx; see the section ‘Indexing terms’ below) write the raw index material to foo.idx.
2. MakeIndex reads foo.idx, collects and sorts the index, and writes the result to foo.ind.
3. Eplain reads and typesets foo.ind on a subsequent run of TeX. See the section ‘Typesetting an index’ below.

The texi2dvi program can help you automate this process (see Invoking Eplain).

If your document needs more than one index, each must have its own file. Therefore, Eplain provides the command \defineindex, which takes an argument that is a single letter, which replaces ‘i’ in the filenames and in the indexing command names described below. For example,

\defineindex{m}

defines the command \mdx to write to the file foo.mdx. Eplain simply does \defineindex{i} to define the default commands.

Note that MakeIndex does not use the above naming scheme for multiple indexes. Unless instructed otherwise, MakeIndex always writes its output to a file with extension .ind. For example, if you define an additional index with the command \defineindex{j}, you’ll need to run MakeIndex like this:

$ makeindex book.jdx -o book.jnd

For each index defined with \defineindex{n}, Eplain provides a switch \ifndx which controls whether indexing commands write index entries to the corresponding index file. However, even when index term writing is disabled, indexing commands still do all other processing of their arguments, including typesetting of proof index terms (see Proofing index terms.

For example, if you write \idxfalse near the beginning of a document foo.tex (before the first indexing command), Eplain will not open the default index file (foo.idx) and the corresponding indexing commands (\idx, \sidx, etc.) will not write index entries there. This may be useful for draft compilations of a manuscript, e.g., to avoid the overhead of index file input/output.

• Indexing terms
• Typesetting an index
• Customizing indexing

truth
  definition of, 75

\sidx{Ap-weight@$A_\pi$-weight}

\hookaction{afterindexterm}{\if@aftersctnhead \nobreak \fi}

\idx{"!}

!\sidx{"!}

\sidx[see]{comments}[with %@with \verbatim %"|endverbatim]
  {commenting with \verbatim %"|endverbatim}

\sidx {braces {, }@braces
  \verbatim {"|endverbatim, \verbatim }"|endverbatim}

\def\idxargopen{`\<}
\def\idxargclose{`\>}
\sidx <left brace "{@left brace \verbatim "{"|endverbatim>

\sidx{entry!subentry with a bracket [}

\hookaction{beginindex}{\triplecolumns}

\hangindent = 1em
\raggedright
\hyphenpenalty = 10000

\hookaction{indexitem}{\advance\rightskip by 2em}

delim_0 "\\afterindexterm "
delim_1 "\\afterindexterm "
delim_2 "\\afterindexterm "

analysis,
  archetypal (see archetypal criticism)
archetypal criticism,
  elements of, 75, 97, 114 (see also dichotomies)

\def\indexsee#1#2{({\seevariant \indexseeword\/ }#1)}
\def\indexseealso#1#2{({\seevariant \indexseealsowords\/ }#1)}

$ cat myfile.idx | makeindex | ./trimsee >myfile.ind

4.13 Justification ¶

Eplain defines three commands to conveniently justify multiple lines of text: \flushright, \flushleft, and \center.

They all work in the same way; let’s take \center as the example. To start centering lines, you say \center inside a group; to stop, you end the group. Between the two commands, each end-of-line in the input file also starts a new line in the output file.

The entire block of text is broken into paragraphs at blank lines, so all the TeX paragraph-shaping parameters apply in the usual way. This is convenient, but it implies something else that isn’t so convenient: changes to any linespacing parameters, such as \baselineskip, will have no effect on the paragraph in which they are changed. TeX does not handle linespacing changes within a paragraph (because it doesn’t know where the line breaks are until the end of the paragraph).

The space between paragraphs is by default one blank line’s worth. You can adjust this space by assigning to \blanklineskipamount; this (vertical) glue is inserted after each blank line.

Here is an example:

{\center First line.

   Second line, with a blank line before.
}

This produces:

You may wish to use the justification macros inside of your own macros. Just be sure to put them in a group. For example, here is how a title macro might be defined:

\def\title{\begingroup\titlefont\center}
\def\endtitle{\endgroup}

In addition, Eplain defines \raggedleft, analogous to plain TeX’s \raggedright. This macro is also typically used inside a group, but unlike the environments above, TeX does normal line breaking; that is, ends-of-lines in the input file aren’t treated specially. Just like plain’s \raggedright, it also resets \spaceskip and \xspaceskip so that interword spacing is uniform. It also sets \parfillskip to zero so that last lines of paragraphs are also “ragged left”. Finally, \leftskip’s new value is taken from a new glue register, \raggedleft; its default value is 0pt plus 2em, the same as \raggedright’s \rightskip.

Here’s an example:

{\raggedleft This text will be set ragged left,
although the left margin won't be too ragged by default.
You may well want to increase the value of
{\tt \char`\\raggedleftskip} before calling the macro.
It's necessary to end the paragraph before ending the group
or the setting won't have any effect, so: {\tt \char`\\par}
}

Despite \raggedleft resetting \parfillskip to zero, TeX’s line breaking may still prefer to make the last line of a paragraph considerably shorter than the rest, to minimize overall badness. Increasing \raggedleftskip may help somewhat, but using \emergencystretch, retaining interword stretchability by assigning \leftskip directly, or even forcing line breaks may be necessary.

4.14 Tables ¶

Eplain provides a single command, \makecolumns, to make generating one particular kind of table easier. More ambitious LaTeX styles and macro packages tackle more difficult applications. The autorows feature of the Memoir package provides similar functionality to this.

Many tables are homogenous, i.e., all the entries are semantically the same. The arrangement into columns is to save space on the page, not to encode different meanings. In this kind of the table, it is useful to have the column breaks chosen automatically, so that you can add or delete entries without worrying about the column breaks.

\makecolumns takes two arguments: the number of entries in the table, and the number of columns to break them into. As you can see from the example below, the first argument is delimited by a slash, and the second by a colon and a space (or end-of-line). The entries for the table then follow, one per line (not including the line with the \makecolumns command itself).

\parindent defines the space to the left of the table. \hsize defines the width of the table. So you can adjust the position of the table on the page by assignments to these parameters, probably inside a group.

You can also control the penalty at a page break before the \makecolumns by setting the parameter \abovecolumnspenalty. Usually, the table is preceded by some explanatory text. You wouldn’t want a page break to occur after the text and before the table, so Eplain sets it to 10000. But if the table produced by \makecolumns is standing on its own, \abovecolumnspenalty should be decreased.

If you happen to give \makecolumns a smaller number of entries than you really have, some text beyond the (intended) end of the table will be incorporated into the table, probably producing an error message, or at least some strange looking entries. And if you give \makecolumns a larger number of entries than you really have, some of the entries will be typeset as straight text, probably also looking somewhat out of place.

Here is an example:

% Arrange 6 entries into 2 columns:
\makecolumns 6/2: % This line doesn't have an entry.
one
two
three
four
five
six
Text after the table.

This produces ‘one’, ‘two’, and ‘three’ in the first column, and ‘four’, ‘five’, and ‘six’ in the second.

4.15 Margins ¶

TeX’s primitives describe the type area in terms of an offset from the upper left corner, and the width and height of the type. Some people prefer to think in terms of the margins at the top, bottom, left, and right of the page, and most composition systems other than TeX conceive of the page laid out in this way. Therefore, Eplain provides commands to directly assign and increment the margins.

\topmargin = dimen ¶

\bottommargin = dimen

\leftmargin = dimen

\rightmargin = dimen

These commands set the specified margin to the dimen given. The = and the spaces around it are optional. The control sequences here are not TeX registers, despite appearances; therefore, commands like \showthe\topmargin will not do what you expect.

\advancetopmargin by dimen ¶

\advancebottommargin by dimen

\advanceleftmargin by dimen

\advancerightmargin by dimen

These commands change the specified margin by the dimen given.

Regardless of whether you use the assignment or the advance commands, Eplain always changes the type area in response, not the other margins. For example, when TeX starts, the left and right margins are both one inch. If you then say \leftmargin = 2in, the right margin will remain at one inch, and the width of the lines (i.e., \hsize) will decrease by one inch.

When you use any of these commands, Eplain computes the old value of the particular margin, by how much you want to change it, and then resets the values of TeX’s primitive parameters to correspond. Unfortunately, Eplain cannot compute the right or bottom margin without help: you must tell it the full width and height of the final output page. It defines two new parameters for this:

\paperheight ¶

The height of the output page; default is 11truein.

\paperwidth ¶

The width of the output page; default is 8.5truein.

If your output page has different dimensions than this, you must reassign to these parameters, as in

\paperheight = 11truein
\paperwidth = 17truein

4.16 Multiple columns ¶

Eplain provides for double, triple, and quadruple column output: say \doublecolumns, \triplecolumns, or \quadcolumns, and from that point on, the manuscript will be set in columns. To go back to one column, say \singlecolumn.

You may need to invoke \singlecolumn to balance the columns on the last page of output.

To do a “column eject”, i.e., move to the top of the next column, do \columnfill. This does not actually force an eject, however: it merely inserts an unbreakable space of (essentially) size \@normalvsize minus \pagetotal (where \@normalvsize is the usual height of the page; to implement multicolumns, Eplain multiplies \vsize itself by the number of columns). In most circumstances, a column break will be forced after this space (during the column splitting operation when the whole page is output).

The columns are separated by the value of the dimen parameter \gutter. Default value is two picas. If you want to add vertical material between the columns, use \gutterbox. For example, to put a vertical line between columns, define \gutterbox as

\def\gutterbox{\vbox to \dimen0{\vfil\hbox{\vrule height\dimen0}\vfil}}%

There are known bugs in the multiple-column code such that \topmark and possibly other marks can have an incorrect value on the last page of multiple-column material when using \singlecolumn to balance the columns. Unfortunately this is quite difficult to fix, and at present (volunteers welcome), it’s going to remain. A suboptimal workaround is to insert \columnfill at the appropriate place.

The dimension counter \dimen0 contains the height of the column.

All the \…columns macros insert the value of the glue parameter \abovecolumnskip before the multicolumn text, and the value of the glue parameter \belowcolumnskip after it. The default value for both of these parameters is \bigskipamount, i.e., one linespace in plain TeX.

The macros take into account only the insertion classes defined by plain TeX; namely, footnotes and \topinserts. If you have additional insertion classes, you will need to change the implementation.

Also, Eplain makes insertions the full page width. There is no provision for column-width insertions.

4.17 Footnotes ¶

The most common reference mark for footnotes is a raised number, incremented on each footnote. The \numberedfootnote macro provides this. It takes one argument, the footnote text.

If your document uses only numbered footnotes, you could make typing \numberedfootnote more convenient with a command such as:

\let\footnote = \numberedfootnote

After doing this, you can type your footnotes as \footnote{footnote text}, instead of as \numberedfootnote{footnote text}.

Eplain keeps the current footnote number in the count register \footnotenumber. So, to reset the footnote number to zero, as you might want to do at, for example, the beginning of a chapter, you could say \footnotenumber=0.

Plain TeX separates the footnote marker from the footnote text by an en space (it uses the \textindent macro). In Eplain, you can change this space by setting the dimension register \footnotemarkseparation. The default is still an en.

You can produce a space between footenotes by setting the glue register \interfootnoteskip. The default is zero.

\parskip is also set to zero by default before the beginning of each footnote (but not for the text of the footnote).

You can also control footnote formatting in a more general way: Eplain expands the token register \everyfootnote before a footnote is typeset, but after the default values for all the parameters have been established. For example, if you want your footnotes to be printed in seven-point type, indented by one inch, you could say:

\everyfootnote = {\sevenrm \leftskip = 1in}

By default, an \hrule is typeset above each group of footnotes on a page. You can control the dimensions of this rule by setting the dimension registers \footnoterulewidth and \footnoteruleheight. The space between the rule and the first footnote on the page is determined by the dimension register \belowfootnoterulespace. If you don’t want any rule at all, set \footenoteruleheight=0pt, and, most likely, \belowfootnoterulespace=0pt. The defaults for these parameters typeset the rule in the same way as plain TeX: the rule is 0.4 points high, 2 true inches wide, with 2.6 points below it.

The space above the rule and below the text on the page is controlled by the glue register \skip\footins. The default is a plain TeX \bigskip.

Eplain can create hypertext links for the footnote marks (see Footnote hyperlinks: foot, footback).

4.18 Fractions ¶

Exercise 11.6 of The TeXbook describes a macro \frac for setting fractions, but \frac never made it into plain TeX. So Eplain includes it.

\frac typesets the numerator and denominator in \scriptfont0, slightly raised and lowered. The numerator and denominator are separated by a slash. The denominator must be enclosed in braces if it’s more than one token long, but the numerator need not be. (This is a consequence of \frac taking delimited arguments; see page 203 of The TeXbook for an explanation of delimited macro arguments.)

For example, \frac 23/{64} turns ‘23/64’ into .

4.19 Paths ¶

When you typeset long pathnames, electronic mail addresses, or other such “computer” names, you would like TeX to break lines at punctuation characters within the name, rather than trying to find hyphenation points within the words. For example, it would be better to break the email address letters@alpha.gnu.ai.mit.edu at the ‘@’ or a ‘.’, rather than at the hyphenation points in ‘letters’ and ‘alpha’.

If you use the \path macro to typeset the names, TeX will find these good breakpoints. The argument to \path is delimited by any character other than ‘\’ which does not appear in the name itself. ‘|’ is often a good choice, as in:

\path|letters@alpha.gnu.ai.mit.edu|

You can control the exact set of characters at which breakpoints will be allowed by calling \discretionaries. This takes the same sort of delimited argument; any character in the argument will henceforth be a valid breakpoint within \path. The default set is essentially all the punctuation characters:

\discretionaries |~!@$%^&*()_+`-=#{}[]:";'<>,.?\/|

If for some reason you absolutely must use \ as the delimiter character for \path, you can set \specialpathdelimiterstrue. (Other delimiter characters can still be used.) TeX then processes the \path argument about four times more slowly.

The \path macro comes from path.sty, written by Nelson Beebe and Philip Taylor, and available at https://ctan.org/pkg/path.

4.20 Logos ¶

Eplain redefines the \TeX macro of plain TeX to end with \null, so that the proper spacing is produced when \TeX is used at the end of a sentence. The other …TeX macros listed here do this, also.

Eplain defines \AMSLaTeX, \AMSTeX, \BibTeX \eTeX, \ExTeX, \LAMSTeX, \LaTeX, \MF, \SLiTeX, \XeLaTeX, and \XeTeX to produce their respective logos. (Sorry, the logos are not shown here.) Some spelling variants of these are also supported.

Most of these macros come from texnames.sty, compiled by Nelson Beebe and available at https://mirror.ctan.org/info/biblio/texnames.sty (part of the biblio package, https://ctan.org/pkg/biblio).

4.21 Boxes ¶

The solid rectangle that Eplain uses as a marker in unordered lists (see Lists) is available by itself: just say \blackbox.

You can create black boxes of arbitrary size with \hrule or \vrule.

You can also get unfilled rectangles with \makeblankbox. This takes two explicit arguments: the height and depth of the rules that define the top and bottom of the rectangle. (The two arguments are added to get the width of the left and right borders, so that the thickness of the border is the same on all four sides.) It also uses, as implicit arguments, the dimensions of \box0 to define the dimensions of the rectangle it produces. (The contents of \box0 are ignored.)

Here is an example. This small raised open box is suitable for putting next to numbers in, e.g., a table of contents.

\def\openbox{%
  \ht0 = 1.75pt \dp0 = 1.75pt \wd0 = 3.5pt
  \raise 2.75pt \makeblankbox{.2pt}{.2pt}
}

Finally, you can put a box around arbitrary text with \boxit. This takes one argument, which must itself be a (TeX) box, and puts a printed box around it, separated by \boxitspace white space (3 points by default) on all four sides. For example:

\boxit{\hbox{This text is boxed.}}

The reason that the argument must be a box is that when the text is more than one line long, TeX cannot figure out the line length for itself. Eplain does set \parindent to zero inside \boxit, since it is very unlikely you would want indentation there. (If you do, you can always reset it yourself.)

\boxit uses \ehrule and \evrule so that you can easily adjust the thicknesses of the box rules. See Rules.

4.22 Checking for PDF output ¶

You might sometimes want to test whether the target format is .pdf or .dvi. The \ifpdf conditional can be used for this:

\ifpdf
   This text is produced when the engine outputs PDF.
\else
   This text is produced when the engine outputs DVI (or similar).
\fi

At this writing, \ifpdf will be true when running pdfTeX or LuaTeX with PDF output. It will be false when running XeTeX, or (of course) original TeX, etc.

Eplain defines \ifpdf by incorporating iftex.sty, a package now maintained by the LaTeX Project Team. iftex.sty, and therefore Eplain, defines numerous related conditionals to test for different engines; see its package documentation for details: https://ctan.org/pkg/ifpdf.

4.23 Loading LaTeX packages ¶

Eplain provides a limited support for loading LaTeX packages (.sty files—not .cls). This will mostly work for packages which were designed with plain TeX compatibility in mind, which means that most LaTeX packages cannot be loaded. The packages which are known to work are listed below (see Packages known to work). If you discover a working package which is not in the list, please report it to the Eplain mailing list (see Introduction).

To set up a pseudo-LaTeX environment for the packages, Eplain uses miniltx.tex (https://ctan.org/pkg/miniltx) from the LaTeX graphics collection, written by David Carlisle and Sebastian Rahtz. Eplain extends miniltx.tex to provide (primarily) support for package options; in many cases, you can use miniltx.tex directly without loading Eplain at all.

• The \usepackage command
• Environment for loading packages
• Packages known to work
• Packages known not to work

\usepackage[options]{packages}[version]

\usepackage[foo,bar]{pack1,pack2}[2005/08/29]

\beginpackages
  \usepackage[foo,bar]{pack1}
  \usepackage{pack2}
\endpackages

\beginpackages
  \usepackage{pack1}
  \let\input\eplaininput
  \usepackage{pack2}
\endpackages

5.1 Introduction to hyperlinks ¶

The original TeX engine has no built-in support for hyperlinks (a.k.a. hypertext links). Many of the present-day file formats with hyperlinking capabilities did not even exist at the time TeX was written. However, TeX’s \special primitive can be used to instruct TeX to write special directives into its .dvi output file. These directives are not interpreted by TeX in any way; they are intended for programs which process the .dvi files produced by TeX, be it printing or converting to other formats, such as .ps or .pdf.

Another approach is to extend the original TeX engine with the ability to generate one of the hyperlinking formats; TeX’s set of primitives can be extended to include hyperlink commands. This is the approach used by the pdfTeX engine, which is capable of producing .pdf files directly from the TeX source, skipping the .dvi generation and processing step.

It turns out that the sets of commands for different formats are mostly not interchangeable, as each of the file formats has its own quirks and capabilities. And this is where Eplain hyperlink drivers come into play.

In order for Eplain to generate proper commands, Eplain has to know two things: which engine or .dvi processor you are using, and the set of commands it understands.

The knowledge about the commands that the various processors understand is programmed into Eplain’s hyperlink drivers. Eplain provides three drivers: hypertex (implementation of the HyperTeX standard, see https://arxiv.org/hypertex), and pdftex and dvipdfm (named after the programs which process the hyperlink commands, pdfTeX and dvipdfm). Therefore, Eplain can only produce HyperTeX commands and hyperlink commands for one of these two programs—except that the extended dvipdfmx program can be used as well as the original dvipdfm, since they are compatible.

To tell Eplain which .dvi processor or extended TeX engine you are using, use the command \enablehyperlinks.

For example:

\enablehyperlinks

instructs Eplain to attempt to automatically detect which driver to use, as follows: if it detects pdfTeX in PDF mode, it loads the pdftex driver. If it does not detect pdfTeX in PDF mode, the hypertex driver is loaded. The detection is based on the \ifpdf switch (see Checking for PDF output).

If necessary, you can explicitly specify the driver name:

\enablehyperlinks[dvipdfm]

will start producing hyperlinks under the assumption that you are using pdfTeX.

Eplain does not produce any hyperlinks until you explicitly enable them with \enablehyperlinks. For one thing, this keeps Eplain backward-compatible with previous releases without hyperlink support. For another, you may be using a program other than pdfTeX or dvipdfm, which does not understand their hyperlink commands or the HyperTeX commands.

Concepts and Terminology ¶

In general, hyperlinks work as follows. You mark some place in your document as a hyperlink destination, associating a hyperlink label with that destination. Next, somewhere within your document, you create a hyperlink, using a label to identify the destination you want this link to point to. A hyperlink is a region in the document (which can take many forms, for example, text or a picture); when a user clicks on that region, they will be taken to a place in the document marked by the corresponding destination. The following two sections (Explicit hyperlinks, and Implicit hyperlinks) describe the macros you can use to define destinations and create links pointing to those destinations.

In the rest of this chapter, we will often need to refer to links and destinations jointly, in which case we will use the term hyperlinks. We will use the terms links and destinations in cases when we need to refer specifically to links or destinations.

Hyperlink drivers provide several kinds of links and destinations. We will refer to them as link types and destination types.

For example, one of the destination types that the pdftex driver provides is the ‘xyz’ type; when the user follows a link pointing to an ‘xyz’ destination, the exact location marked by that destination is displayed. Another destination type provided by the pdftex driver is the ‘fit’ type; when the user follows a link pointing to a ‘fit’ destination, the page containing that destination is zoomed to fit into the window in which the document is displayed.

Similarly, drivers support various link types. For example, with the pdftex driver, the usual link type used to refer to destinations in the current document is called ‘name’. You can also create a link pointing to another local document (by using the ‘filename’ link type) or to a url (by using the ‘url’ link type).

In addition, each hyperlink driver supports a number of destination and link options. By setting these options you can customize hyperlink parameters (e.g., the thickness of the border drawn around a hyperlink) or pass information to hyperlinks (for example, file name of a document, for a link pointing to a destination in another document).

See Hyperlink drivers, for the description of hyperlink types and options supported by the drivers. See Setting hyperlink types and options, for the information on how to set hyperlink options.

5.2 Explicit hyperlinks ¶

Explicit hyperlinks are created by you, in the source of your document. The simplest command is \hldest, which marks the current position in your document as a destination:

\hldest{type}{options}{label}

Here type is one of the destination types supported by the hyperlink driver (see Hyperlink drivers), options is a comma-separated list of option assignments, and label is the hyperlink label to associate with this destination. This label will identify the destination when creating links pointing to this destination. For example, with the pdftex driver, the command

\hldest{xyz}{zoom=2000}{index}

creates a destination of type ‘xyz’ (“the current position”), sets the magnification ratio for this destination to be 200%, and associates the label index with the destination.

Another command, \hlstart, paired with \hlend, turns all intervening material into a link:

\hlstart{type}{options}{label} ... \hlend

Here type, options and label have the same meaning as for \hldest. Continuing the previous example,

\hlstart{name}{bstyle=U,bwidth=2}{index} Index\hlend

typesets the word ‘Index’ as a link with underline border of width 2 PostScript points, pointing to the named destination index defined in the previous example. (The other options, like highlight mode and border color, are determined by the defaults, see Setting default types and options).

The label argument of both \hldest and \hlstart can contain special characters (such as ‘#’, ‘%’, ‘&’, ‘~’, etc.) without any escaping. This is especially important for url links supported by some drivers (see Hyperlink drivers).

Both \hldest and \hlstart ignore following spaces.

Both \hldest and \hlstart expand the first token of options once, so you can save a list of options in a macro and pass it for the options. For example:

\def\linkopts{bstyle=U,bwidth=2}
\hlstart{name}{\linkopts}{index}Index\hlend

is functionally equivalent to the previous example.

See Hyperlinks (xhyper.tex), for a demonstration of the usage of explicit hyperlinks.

5.3 Implicit hyperlinks ¶

Implicit hyperlinks are hyperlinks created implicitly by various Eplain macros, such as the macros for citations, cross-references, indexing, etc.

All such macros are divided into link groups and destination groups (or linkgroups and destgroups for short) so that parameters can be set individually for each group. For example, all equation macros which define a destination are assigned to the ‘eq’ destgroup; equation macros which create a link are assigned to the ‘eq’ linkgroup. By setting parameters for the ‘eq’ linkgroup (destgroup), you can uniformly customize all links (destinations) related to equation references, without interfering with settings for the other groups.

See Setting hyperlink types and options, for information on how to set parameters for a group.

Here is the list of the linkgroups:

hrefint, hrefext, url, cite, ref, xref, eq, idx, foot, footback.

And here are the destgroups:

bib, li, definexref, xrdef, eq, idx, foot, footback.

See Hyperlinks (xhyper.tex), for a demonstration of the usage of implicit hyperlinks.

The following subsections describe each of the linkgroups and destgroups and the hyperlink support provided.

• General hyperlinks: hrefint, hrefext
• URL hyperlinks: url
• Citation hyperlinks: cite, bib
• List hyperlinks: li
• Cross-reference hyperlinks: definexref, ref
• Page reference hyperlinks: xrdef, xref
• Equation reference hyperlinks: eq
• Index hyperlinks: idx
• Footnote hyperlinks: foot, footback
• Contents hyperlinks

See \href{#intro}{Introduction}

\href{https://tug.org/eplain/doc/eplain.html#Hyperlinks}{Hyperlinks
  in Eplain}
\href{mailto:tex-eplain@tug.org}{Eplain mailing list}

\href{#WeirdChars}{The weird chars \verbatim #&%$~|endverbatim}

\input eplain
\beginpackages
  \usepackage{url}
\endpackages

\enablehyperlinks

\url{https://foo/bar}

\url{mailto:foobar@example.org}

\bye

\input eplain
\beginpackages
  \usepackage{url}
  \usepackage{color}
\endpackages

\enablehyperlinks
\hlopts{bwidth=0}

\url{https://foo/bar}

\url{mailto:foobar@example.org}

\bye

\indexentry{entry|pagemarkup}{pageno}

\indexentry{entry|hlidx{label}{cs}}{pageno}

\hlidx{label}{cs}{pageno}

$ ./idxuniq file.idx | makeindex >file.ind

\indexentry{entry|hlidxpage{cs}}{pageno}

\hlidxpage{cs}{pageno}

\setidxpagelistdelimiter{list-delim}
\setidxpagerangedelimiter{page-delim}

\enablehyperlinks[idxexact]

\sidx{semantic theory of truth@%
      \leavevmode\hldest{}{}{idx:theo truth}semantic theory of truth}
...
\sidx[seealso]{truth}[definition of]%
      {\hlstart{}{}{idx:theo truth}semantic theory of truth\hlend}

\hldeston[foot,footback]
\hlon[foot,footback]

5.4 Hyperlink drivers ¶

This section describes the hyperlink drivers: the types of hyperlinks they support, and the options they accept. During the first reading, you may only want to skim through this section.

Some of the descriptions below come from Portable Document Format Reference Manual Version 1.3, March 11, 1999.

• Options supported by all drivers
• Hyperlink driver hypertex
• Hyperlink drivers pdftex and dvipdfm
• Hyperlink driver nolinks

https://arxiv.org/hypertex
https://mirror.ctan.org/support/hypertex/hypertex

\ifpdf
  \enablehyperlinks
\else
  \enablehyperlinks[nolinks]%
\fi

5.5 Setting hyperlink types and options ¶

You can define default types for links and destinations, which will be used when you do not specify a type in \hlstart or \hldest. Similarly, you can define default values for the options; the default value for an option is used when you do not set the option in the argument to \hlstart or \hldest.

The parameters for implicit links and destinations can be customized by setting the “group” types and options. When not set, the defaults are used.

All these settings are local to the current (TeX) group, so if you want to set an option temporarily, you can do so inside a \begingroup…\endgroup block; when the group ends, the previous settings are restored.

• Setting default types and options
• Setting group types
• Setting group options

\hltype{type}
\hldesttype{type}

\hlopts{options}
\hldestopts{options}

\hlopts{bwidth=0}

\hltype[groups]{type}
\hldesttype[groups]{type}

\hltype[eq,]{type}

\hldesttype[*,]{type}

\hlopts[groups]{options}
\hldestopts[groups]{options}
\hlopts![groups]{options}
\hldestopts![groups]{options}

$$\hldestopts[eq]{raise=2.5\normalbaselineskip}
...
$$

raise=1.7\normalbaselineskip

5.6 Turning hyperlinks on/off ¶

Links and/or destinations can be turned on or off globally by disabling the low-level commands, or for each group individually.

All these settings are local to the current (TeX) group, so if you want to enable or disable links/destinations temporarily, you can do so inside a \begingroup…\endgroup block; when the group ends, the previous settings are restored.

• Turning low-level commands on/off
• Turning hyperlinks on/off for a group

\hldeston
\hldestoff
\hlon
\hloff

\hldeston[groups]
\hldestoff[groups]
\hlon[groups]
\hloff[groups]

\hloff
\hloff[eq]
\hlon

5.7 Making PDF outlines ¶

PDF outlines (a.k.a. bookmarks) are more or less a table of contents that PDF viewers can display alongside the main document. Eplain’s hyperlink features can be used to create them; there isn’t any special support for them. A continuing example interspersed with commentary follows.

First we must enable hyperlinks.

\input eplain
\enablehyperlinks %[dvipdfm] doesn't work

We will separate the code to support pdftex from dvips with the \ifpdf conditional (provided by Eplain).

For pdftex, we can use the \pdfoutline primitive. The keyword "count" is followed by the number of subentries in this entry. If negative, the bookmark is closed (that is, subentries are hidden).

\ifpdf
  \pdfoutline goto name {sec1} count -1 {Mysec-pdf}%
  \pdfoutline goto name {sec1.1} {Mysubsec-pdf}%

For dvips, we use TeX’s \special command to emit a ps: special using the PDF pdfmark operator. The ps: prefix tells dvips that the following is literal PostScript.

[ ... pdfmark (there is no closing ‘]’) is a extension to the PostScript language for specifying various PDF-related things. It is recognized by Ghostscript, Distiller, et al. Adobe publishes a reference manual for it: https://adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdfmark_reference.pdf.

The /DOCVIEW pdfmark used here says the outline panel should be used.

\else % not pdf output
  \special{ps:[/PageMode /UseOutlines /DOCVIEW pdfmark}
  %
  % The individual outline entries, using a different syntax
  % than pdftex, but the same information.
  \special{ps:[/Count -1 /Dest (sec1) cvn /Title (Mysec-dvi)
                /OUT pdfmark}
  \special{ps:[/Count -0 /Dest (sec1.1) cvn /Title (Mysubsec-dvi)
                /OUT pdfmark}
\fi

The ‘-pdf’ and ‘-dvi’ suffixes in the strings above in the outline entries are just to make it clear which branch is being executed, for purposes of this example. Ordinarily the entries would be the same in both branches.

Also, the strings above are literal PostScript constants, again for this example. Usually they would come from control sequences, e.g., as the table of contents is read.

It is necessary to "pdf-escape" such arbitrary strings, else backslashes, parentheses, etc., would not come out right. pdfTeX’s \pdfescapestring primitive is an easy way to do this, e.g., \xdef#1{\pdfescapestring{#1}}.

Here is the document text, constructing three pages with the section and subsection given above in the outlines.

First page.\vfil\eject

\hldest{}{}{sec1}%
1. Mysec on second page.\vfil\eject

\hldest{}{}{sec1.1}%
1.1. Mysubsec on third page.

\end

6.1 Slanted lines and vectors ¶

The macros \drawline and \drawvector provide the capability found in LaTeX’s picture mode to draw slanted lines and vectors of certain directions. Both of these macros take three arguments: two integer arguments to specify the direction of the line or vector, and one argument to specify its length. For example, ‘\drawvector(-4,1){60pt}’ produces the vector which lies in the 2d quadrant, has a slope of minus 1/4, and a width of 60 pt.

Note that if an \hbox is placed around \drawline or \drawvector, then the width of the \hbox will be the positive dimension specified in the third argument, except when a vertical line or vector is specified, e.g., \drawline(0,1){1in}, which has zero width. If the specified direction lies in the 1st or 2d quadrant (e.g., (1,1) or (-2,3)), then the \hbox will have positive height and zero depth. Conversely, if the specified direction lies in the 3d or 4th quadrant (e.g., (-1,-1) or (2,-3)), then the \hbox will have positive depth and zero height.

There are a finite number of directions that can be specified. For \drawline, the absolute value of each integer defining the direction must be less than or equal to six, i.e., (7,-1) is incorrect, but (6,-1) is acceptable. For \drawvector, the absolute value of each integer must be less than or equal to four. Furthermore, the two integers cannot have common divisors; therefore, if a line with slope 2 is desired, say (2,1) instead of (4,2). Also, specify (1,0) instead of, say, (3,0) for horizontal lines and likewise for vertical lines.

Finally, these macros depend upon the LaTeX font line10. If your site doesn’t have this font, ask your system administrator to get it. Future enhancements will include macros to draw dotted lines and dotted vectors of various directions.

6.2 Commutative diagrams ¶

The primitive commands \drawline and \drawvector can be used to typeset arrow theoretic diagrams. This section describes (1) macros to facilitate typesetting arrows and morphisms, and (2) macros to facilitate the construction of commutative diagrams. All macros described in this section must be used in math mode.

• Arrows and morphisms
• Construction of commutative diagrams
• Commutative diagram parameters

$$\commdiag{Y&\mapright^f&E\cr \mapdown&\arrow(3,2)\lft{f_t}&\mapdown\cr
Y\times I&\mapright^{\bar f_t}&X}$$

$$\varrowlength=20pt
\commdiag{V\otimes W\cr \mapup\lft\phi&\arrow(3,-1)\rt{\tilde l}\cr
V\times W&\mapright^l&U\cr}$$

$$\sarrowlength=.42\harrowlength
\commdiag{&R^m\cr &\arrow(-1,-1)\lft{\bf B}\quad \arrow(1,-1)\rt{\bf G}\cr
R^n&\mapright^{\bf P}&R^n\cr
\mapdown\lft{e^{{\bf A}t}}&&\mapdown\rt{e^{{\bf F}t}}\cr
R^n&\mapright^{\bf P}&R^n\cr
&\arrow(1,-1)\lft{\bf C}\quad \arrow(-1,-1)\rt{\bf H}\cr
&R^q\cr}$$

$$\harrowlength=48pt \varrowlength=48pt \sarrowlength=20pt
\def\cross#1#2{\setbox0=\hbox{$#1$}%
  \hbox to\wd0{\hss\hbox{$#2$}\hss}\llap{\unhbox0}}
\gridcommdiag{&&B&&\mapright^b&&D\cr
&\arrow(1,1)\lft a&&&&\arrow(1,1)\lft d\cr
A&&\cross{\hmorphposn=12pt\mapright^c}{\vmorphposn=-12pt\mapdown\lft f}
&&C&&\mapdown\rt h\cr\cr
\mapdown\lft e&&F&&\cross{\hmorphposn=-12pt\mapright_j}
{\vmorphposn=12pt\mapdown\rt g}&&H\cr
&\arrow(1,1)\lft i&&&&\arrow(1,1)\rt l\cr
E&&\mapright_k&&G\cr}$$

$$\hgrid=16pt \vgrid=8pt \sarrowlength=32pt
\def\cross#1#2{\setbox0=\hbox{$#1$}%
  \hbox to\wd0{\hss\hbox{$#2$}\hss}\llap{\unhbox0}}
\def\l#1{\llap{$#1$\hskip.5em}}
\def\r#1{\rlap{\hskip.5em$#1$}}
\gridcommdiag{&&U&&&&V\cr &&\bullet&&&&\bullet\cr
&&\sarrowlength=16pt\sline(0,1)&&&&\sarrowlength=16pt\sline(0,1)\cr
&&\l{u(U\cap V)}\bullet&&&&\bullet\r{(U\cap V)v}\cr
&&&\sline(2,-1)&&\sline(2,1)\cr
&&\cross{=}{\sline(0,1)}&&\bullet&&\cross{=}{\sline(0,1)}\cr\cr
&&\l{^{\textstyle u(U\cap v)}}\bullet&&\cross{=}{\sline(0,1)}&&
 \bullet\r{^{\textstyle(u\cap V)v}}\cr
&\sline(2,1)&&\sline(2,-1)&&\sline(2,1)&&\sline(2,-1)\cr
\l{u}\bullet&&&&\bullet&&&&\bullet\r{v}\cr
&\sline(2,-1)&&\sline(2,1)&&\sline(2,-1)&&\sline(2,1)\cr
&&\bullet&&&&\bullet\cr &&u\cap V&&&&U\cap v\cr}$$

7.1 Category codes ¶

Plain TeX defines \active (as the number 13) for use in changing category codes. Although the author of The TeXbook has “intentionally kept the category codes numeric”, two other categories are commonly used: letters (category code 11) and others (12). Therefore, Eplain defines \letter and \other.

Sometimes it is cleaner to make a character active without actually writing a \catcode command. The \makeactive command takes a character as an argument to make active (and ignores following spaces). For example, here are two commands which both make \ active:

\makeactive\\   \makeactive92

Sometimes you might want to temporarily change the category code of the ‘@’ character to \letter, so that you can use or define macros which are normally inaccessible to the user. For such situations, Eplain provides the \makeatletter command. It sets the category code of ‘@’ to \letter (11) and defines \resetatcatcode to restore the category code to whatever it was before the call to \makeatletter. For example:

\makeatletter
\def\@hidden@macro{This macro cannot normally be
                   called / redefined by the user}
\resetatcatcode

There is also \makeatother which works similarly but sets the category code of ‘@’ to \other (12).

Usually, when you give a definition to an active character, you have to do so inside a group where you temporarily make the character active, and then give it a global definition (cf. the definition of \obeyspaces in The TeXbook). This is inconvenient if you are writing a long macro, or if the character already has a global definition you do not wish to transcend. Eplain provides \letreturn, which defines the usual end-of-line character to be the argument. For example:

\def\mymacro{... \letreturn\myreturn ... }
\mymacro hello
there

The end-of-line between ‘hello’ and ‘there’ causes \myreturn to be expanded.

The TeXbook describes \uncatcodespecials, which makes all characters which are normally “special” into “other” characters, but the definition never made it into plain TeX. Eplain therefore defines it. For notes on the usage, see Verbatim listing.

Finally, \percentchar expands into a literal ‘%’ character. This is useful when you \write TeX output to a file, and want to avoid spurious spaces. For example, Eplain writes a \percentchar after the definition of cross-references. The macros \lbracechar and \rbracechar expand similarly.

7.2 Allocation macros ¶

Plain TeX provides macros that allocate registers of each primitive type in TeX, to prevent different sets of macros from using the same register for two different things. The macros are all named starting with ‘new’, e.g., \newcount allocates a new “count” (integer) register. Such allocations are usually needed only at the top level of some macro definition file; therefore, plain TeX makes the allocation registers \outer, to help find errors. (The error this helps to find is a missing right brace in some macro definition.)

Sometimes, however, it is useful to allocate a register as part of some macro. An outer control sequence cannot be used as part of a macro definition (or in a few other contexts: the parameter text of a definition, an argument to a definition, the preamble of an alignment, or in conditional text that is being skipped). Therefore, Eplain defines “inner” versions of all the allocation macros, named with the prefix ‘inner’: \innernewbox, \innernewcount, \innernewdimen, \innernewfam, \innernewhelp, \innernewif, \innernewinsert, \innernewlanguage, \innernewread,
\innernewskip, \innernewtoks, \innernewwrite.

You can also define non-outer versions of other macros in the same way that Eplain defines the above. The basic macro is called \innerdef:

\innerdef \innername {outername}

The first argument (\innername) to \innerdef is the control sequence that you want to define. Any previous definition of \innername is replaced. The second argument (outername) is the characters in the name of the outer control sequence. (You can’t use the actual control sequence name, since it’s outer!)

If the outer control sequence is named \cs, and you want to define innercs as the inner one, you can use \innerinnerdef, which is just an abbreviation for a call to \innerdef. For example, these two calls are equivalent:

\innerdef\innerproclaim{proclaim}
\innerinnerdef{proclaim}

• Scratch registers

7.3 Iteration ¶

You can iterate through a comma-separated list of items with \for. Here is an example:

\for\name:=karl,kathy\do{%
   \message{\name}%
}%

This writes ‘karl’ and ‘kathy’ to the terminal. Spaces before or after the commas in the list, or after the :=, are not ignored. To strip leading spaces off the items, use \For:

\For\name:=
   karl,
   kathy\do{%
      \message{\name}%
   }%

Note that trailing spaces are still not ignored.

Both \for and \For expand the first token of the item list fully, so this is equivalent to the above:

\def\namelist{karl,kathy}%
\for\name:=\namelist\do ...

However, this won’t work, either with \for or with \For:

\def\namelist{karl,kathy}%
\For\name:= \namelist\do ...

because \for and \For expand the first token after := which is space, not \namelist.

Eplain provides another kind of loops, which is an extension of plain TeX’s \loop. If you say:

\loop
  loop-text
\if condition
  if-text
\repeat

then loop-text will be repeated as long as condition is satisfied (\if can be any of the TeX’s conditional commands, without the matching \fi). Eplain extends this with the optional else clause:

\loop
  loop-text
\if condition
  if-text
\else
  else-text
\repeat

Here, loop-text will be repeated as long as condition is not satisfied. This extension is from Victor Eijkhout’s TeX by Topic (page 104).

7.4 Macro arguments ¶

It is occasionally useful to redefine a macro that takes arguments to do nothing. Eplain defines \gobble, \gobbletwo, and \gobblethree to swallow one, two, and three arguments, respectively.

For example, if you want to produce a “short” table of contents—one that includes only chapters, say—the easiest thing to do is read the entire .toc file (see Contents), and just ignore the commands that produce section or subsection entries. To be specific:

\let\tocchapterentry = \shorttocchapter
\let\tocsectionentry = \gobbletwo
\let\tocsubsectionentry = \gobbletwo
\readtocfile

(Of course, this assumes you only have chapters, sections, and subsections in your document.)

In addition, Eplain defines \eattoken to swallow the single following token, using \let. Thus, \gobble followed by ‘{…}’ ignores the entire brace-enclosed text. \eattoken followed by the same ignores only the opening left brace.

Eplain defines a macro \identity which takes one argument and expands to that argument. This may be useful if you want to provide a function for the user to redefine, but don’t need to do anything by default. (For example, the default definition of \eqconstruct (see Formatting equation references) is \identity.)

You may also want to read an optional argument. The established convention is that optional arguments are put in square brackets, so that is the syntax Eplain recognizes. Eplain ignores space tokens before and after optional arguments, via \futurenonspacelet.

You test for an optional argument by using \@getoptionalarg. It takes one argument, a control sequence to expand after reading the argument, if present. If an optional argument is present, the control sequence \@optionalarg expands to it; otherwise, \@optionalarg is \empty. You must therefore have the category code of @ set to 11 (letter). Here is an example:

\catcode`@=\letter
\def\cmd{\@getoptionalarg\finishcmd}
\def\finishcmd{%
  \ifx\@optionalarg\empty
    % No optional argument present.
  \else
    % One was present.
  \fi
}

It’s possible to define macros that appear to accept optional arguments intermixed with mandatory arguments in any imaginable way. For example:

\makeatletter
% \mo{m}[o]
\def\mo#1{\def\mo@arg{#1}\@getoptionalarg\fin@mo}
\def\fin@mo{\vskip1pc
  ARG:    \mo@arg       \par
  OPTARG: \@optionalarg \par
}
% \mom{m}[o]{m}
\def\mom#1{\def\mom@arg{#1}\@getoptionalarg\fin@mom}
\def\fin@mom#1{\vskip1pc
  ARG1:   \mom@arg      \par
  OPTARG: \@optionalarg \par
  ARG2:   #1\par
}
% \omo[o]{m}[o]
\def\omo{\@getoptionalarg\fin@omo}
\def\fin@omo#1{\let\omo@optarg\@optionalarg
               \def\omo@arg{#1}\@getoptionalarg\@fin@omo}
\def\@fin@omo{\vskip1pc
  OPTARG1: \omo@optarg   \par
  ARG:     \omo@arg      \par
  OPTARG2: \@optionalarg \par
}
\makeatother

If an optional argument contains another optional argument, the inner one will need to be enclosed in braces, so TeX does not mistake the end of the first for the end of the second.

7.5 Converting to characters ¶

Eplain defines \xrlabel to produce control sequence names for cross-reference labels, et al. This macro expands to its argument with an ‘_’ appended. (It does this because the usual use of \xrlabel is to generate a control sequence name, and we naturally want to avoid conflicts between control sequence names.)

Because \xrlabel is fully expandable, to make a control sequence name out of the result you need only do

\csname \xrlabel{label}\endcsname

The \csname primitive makes a control sequence name out of any sequence of character tokens, regardless of category code. Labels can therefore include any characters except for ‘\’, ‘{’, ‘}’, and ‘#’, all of which are used in macro definitions themselves.

\sanitize takes a control sequence as an argument and converts the expansion of the control sequence into a list of character tokens. This is the behavior you want when writing information like chapter titles to an output file. For example, here is part of the definition of \writenumberedtocentry; #2 is the title that the user has given.

...
\def\temp{#2}%
...
  \write\tocfile{%
    ...
    \sanitize\temp
    ...
  }%

7.6 Expansion ¶

This section describes some miscellanous macros for expansion, etc.

• \csn and \ece
• \edefappend
• Hooks
• Properties
• \expandonce
• \ifundefined
• \ifempty
• \ifinteger and \isinteger
• \futurenonspacelet

\expandafter token \csname name \endcsname

\def\fontabbrevdef#1#2{\ece\def{@#1font}{#2}}
\fontabbrevdef{normal}{ptmr}

\def\foo{abc}
\def\bar{xyz}
\edefappend\foo{\bar karl}

\def\begin#1{ ... \hookrun{begin} ... }
\def\end#1{ ... \hookrun{end} ... }
\hookprepend{begin}\start_underline
\hookappend{end}\finish_underline

\setproperty{a}{pr}{xyz}
\getproperty{a}{pr}

\def\foo{abc}
\def\bar{\foo}
\edef\temp{\expandonce\bar}

\def\foo#1{\ifempty{#1} t \else f \fi}

\def\empty{}
\ifempty\empty\message{empty}\else\message{not empty}\fi

\if\isinteger{arg} t \else f \fi

\ifinteger{12_ab}integer\else not integer\fi

\futurenonspacelet\temp\finishup
\def\finishup{\ifx\temp ...}

7.7 Obeying spaces ¶

\obeywhitespace makes both end-of-lines and space characters in the input be respected in the output. Unlike plain TeX’s \obeyspaces, even spaces at the beginnings of lines turn into blank space.

By default, the size of the space that is produced by a space character is the natural space of the current font, i.e., what \ produces.

Ordinarily, a blank line in the input produces as much blank vertical space as a line of text would occupy. You can adjust this by assigning to the parameter \blanklineskipamount: if you set this negative, the space produced by a blank line will be smaller; if positive, larger.

Tabs are not affected by this routine. In particular, if tabs occur at the beginning of a line, they will disappear. (If you are trying to make TeX do the “right thing” with tabs, don’t. Use a utility program like expand instead.)

7.8 Writing out numbers ¶

\numbername produces the written-out form of its argument, i.e., ‘zero’ through ‘ten’ for the numbers 0–10, and numerals for all others.

7.9 Mode-specific penalties ¶

TeX’s built-in \penalty command simply appends to the current list, no matter what kind of list it is. You might intend a particular penalty to always be a “vertical” penalty, however, i.e., appended to a vertical list. Therefore, Eplain provides \vpenalty and \hpenalty which first leave the other mode and then do \penalty.

More precisely, \vpenalty inserts \par if the current mode is horizontal, and \hpenalty inserts \leavevmode if the current mode is vertical. (Thus, \vpenalty cannot be used in math mode.)

7.10 Auxiliary files ¶

It is common to write some information out to a file to be used on a subsequent run. But when it is time to read the file again, you only want to do so if the file actually exists. \testfileexistence is given an argument which is appended to \jobname, and sets the conditional \iffileexists appropriately. For example:

\testfileexistence{toc}%
\iffileexists
   \input \jobname.toc
\fi

\testfileexistence takes an optional parameter; when given, it will override \jobname for the root part of the file name. For example, if you want to test for the file answers.aux, you can do this with the following:

\testfileexistence[answers]{aux}%
\iffileexists
   \input answers.aux
\fi

7.11 User-defined environments ¶

Plain TeX does not provide “named” block structures, only the anonymous \begingroup and \endgroup pair. The disadvantage of this is that when there are several such groups and one is mismatched, it can be difficult to find the error. Eplain provides a named block structure so that if you forget an \environment or an \endenvironment, you will (probably) get an error message about it.

For example:

\def\itpar{
  \environment{@italicpar}
  \it\par
}
\def\enditpar{
  \par
  \endenvironment{@italicpar}%
}

which could then be used to set italicized paragraphs:

\itpar
If I reprehend anything in this world, it is the use of my oracular
tongue, and a nice derangement of epitaphs! 
\enditpar

The above sort of environment allows nesting. But environments shouldn’t always be allowed to nest. Put the control sequence \checkenv at the beginning of a macro that is going to define an environment that should not be nested.

7.12 Page list and page range parsers ¶

The macros which Eplain uses to parse the page lists and ranges in the index, \idxparselist and \idxparserange (see Page destinations for index terms), are sometimes useful when defining page number encapsulators. They take one argument, text to parse. When a page list (range) is not present, they set \idxpagei to be \empty; when a list (range) is detected, they set \idxpagei and \idxpageii to the first and the second page numbers, respectively.

Eplain’s defaults for the page list and page range delimiters are the same as those in MakeIndex, a comma followed by a space (‘, ’) and two dashes (‘--’), respectively. If you customize MakeIndex to use different delimiters, you must not forget to let Eplain know about them with the commands

\setidxpagelistdelimiter{list-delim}
\setidxpagerangedelimiter{page-delim}

These commands save the list-delim and page-delim delimiters in \idxpagelistdelimiter and \idxpagerangedelimiter, respectively.

For example, you may want to define a page number markup command which italicizes and properly underlines page ranges by underlining only the page numbers and not the delimiter:

\def\ituline#1{%
  {\it
  \idxparserange{#1}%
  \ifx\idxpagei\empty
    % The argument is a single page number.
    \underbar{#1}%
  \else
    % The argument is a page range.
    \underbar{\idxpagei}\idxpagerangedelimiter\underbar{\idxpageii}%
  \fi}%
}

Note that the \ituline macro is not aware of page lists. This is not needed if you use hyperlinks in the index, because \hlidx and \hlidxpage will break up the page lists before calling the user’s page encapsulator (see Page destinations for index terms), so \ituline will never see the lists. If, however, you need to design a macro which also takes care of the lists, you can extend \ituline with an additional call to \idxparselist.

8.1 Hyperlinks (xhyper.tex) ¶

% $Id: xhyper.tex 60 2022-10-05 22:42:54Z karl $
% (This file is public domain.)
%
% This file demonstrates the following features of Eplain:
%
%   - explicit and implicit hyperlinks;
%   - symbolic cross-references;
%   - inserting external graphics using |\includegraphics| from
%     the \LaTeX\ package |graphicx.sty|.
%   - rotating text using |\rotatebox| from |graphicx.sty|.
%
% The compiled demo can be downloaded from
%
%   https://tug.org/eplain/demo
%
% In order to compile this file yourself, you will need the CTAN lion
% drawing by Duane Bibby from
%
%   ftp://tug.ctan.org/ftpmaint/CTAN_lion/ctan_lion_350x350.png
%
% (thanks, |www.ctan.org|).  Place the image file in the same
% directory with this file, and change to that directory.  Now, to
% produce a PDF, run twice the command
%
%   pdftex xhyper.tex
%
% During the first run, Eplain will write the information about the
% cross-references into |xhyper.aux|, and during the second run this
% information will be read by Eplain to typeset the references.
%
% Demo case:
%
%   Suppose you are using pdf\TeX, have a figure you want to insert
%   scaled to $200\,pt$ horizontally, and you want this figure to
%   completely fill the PDF viewer's window whenever the reader
%   selects a link pointing to this figure.  Additionally, you want to
%   typeset some text as live hyperlinks, clicking on which will start
%   a Web browser and open a URL.

\input eplain

% Load \LaTeX\ packages.
\beginpackages
  % |url.sty| provides the |\url| command which we will use to typeset
  % a URL.  We request that |url.sty| be the version from June~27,
  % 2005, or later, because earlier versions had problems interacting
  % with plain \TeX.
  \usepackage{url}[2005/06/27]
  % |color.sty| provides support for colored text; all hyperlinks are
  % automatically colored by Eplain when this package is loaded.  We give
  % the |dvipsnames| option because we want to use named colors from the
  % |dvips| graphics driver.
  \usepackage[dvipsnames]{color}
  % Finally, we load |graphicx.sty|, for the macros |\includegraphics|
  % and |\rotatebox|.
  \usepackage{graphicx}
\endpackages

% Remember that hyperlinks are off by default.  Therefore, we need to
% enable them.
\enablehyperlinks

% Customize some hyperlink options.  First, we set border width to~$0$
% for all links to get rid of the default boxes around links (we
% prefer colored links over the boxed links). Next, we say that all
% links created by the |url| hyperlink group (which means the |\url|
% command from |url.sty|) must be colored using the named color
% |BlueViolet|.
\hlopts{bwidth=0}
\hlopts[url]{colormodel=named,color=BlueViolet}

% Inhibit page numbering (we have only one page).
\nopagenumbers

% Define a class word for the cross-reference class |figure|.  This
% word, when defined, will be automatically prepended by Eplain to the
% reference created by |\ref| (read on to see its use).
\def\figureword{fig.}

% Allocate a count register for the figure's number, and a box
% register which we'll use to measure dimensions of the image.
\newcount\fignumber
\newbox\imgbox

% Now comes the fun part--constructing the figure for the image of the
% \CTAN\ lion.  We define a macro
%
%   \fig{LABEL}{FILENAME}{CAPTION}{WIDTH}
%
% which creates a figure using LABEL for the cross-reference and
% hyperlink label, reading the image from file FILENAME, using CAPTION
% to name the figure, and WIDTH to scale the image horizontally.
\def\fig#1#2#3#4{%
  % Leave some space above the figure.  This will also ensure that we
  % are in the vertical mode before we produce a |\vbox|.
  \medskip
  % Advance the figure number.  |\global| ensures that the change to
  % the count register is global, even when |\fig| is buried inside a
  % group.
  \global\advance\fignumber by 1
  % We use |\includegraphics| (from |graphicx.sty|) to load the image,
  % scaled to the specified width, into our ``measuring'' box
  % |\imgbox|.
  \setbox\imgbox = \hbox{\includegraphics[width=#4]{#2}}%
  % To make the demo even more exciting, let's put the figure's
  % caption to the left of the image into the |\indent| space of the
  % new paragraph, and rotate the caption~$90^\circ$.
  \textindent{%
    \rotatebox{90}{F{\sc IGURE}~\the\fignumber.  #3}%
  }%
  % Continue the paragraph by constructing a |\vbox| with the image of
  % the lion.  We use |\definexref| to define the cross-reference
  % label.
  \vbox{%
    % In addition to the cross-reference label, |\definexref| will
    % create a hyperlink destination with the same label.  Therefore,
    % we customize this destination to do what we need.  We say that
    % destination type for the hyperlink group |definexref| (to which
    % |\definexref| belongs) should be |fitr|.  This destination type
    % will magnify the rectangle specified by the options |width|,
    % |height| and |depth| to the PDF viewer's window.  Therefore, we
    % set those options accordingly with |\hldestopts| (notice the use
    % of |depth| instead of |height|---we will want the rectangle to
    % extend {\it downward}, to cover the image which will come
    % next).  Notice that these settings will be isolated within the
    % current group (i.e., the |\vbox| we're constructing).
    \hldesttype[definexref]{fitr}%
    \hldestopts[definexref]{width=\wd\imgbox,height=0pt,depth=\ht\imgbox}%
    % We define a symbolic label so that we can later refer
    % to the figure with |\ref|.  The command |\definexref| does
    % exactly that.  The last argument to |\definexref| specifies
    % class of the label, which determines the word used by |\ref| in
    % front of the reference text (remember that we've defined
    % |\figureword| above).
    \definexref{#1}{\the\fignumber}{figure}%
    % Finally, produce the image which we've been stashing in the box
    % register |\imgbox|.
    \box\imgbox
  }%
  \medskip
}

% Create the figure.
\fig{CTANlion}{ctan_lion_350x350}{Lion in the archives}{200pt}

% Finished with the fun part, we can relax and typeset some
% hyperlinks.  The easiest way to do that is to use the |\ref|
% cross-reference command.  We can even pass an optional argument
% (|the lion in|), which will be placed before the class word (|fig.|)
% and become part of the link (to make sure the reader does not have
% to aim too hard).
Show me \ref[the lion in]{CTANlion}.

% If you are the restless kind, here's another way to create a
% hyperlink to the image:  we create a link explicitly by using
% |\hlstart ... \hlend|.  We don't specify the link type, therefore
% the default type |name| will be used (these are ``named links'',
% i.e., links pointing to destinations identified by labels).  In the
% options argument, we specify that the border of the link should be
% inverted when the user clicks the link (|hlight=O|), and also set
% special color for this link, overriding the default dark-red.  The
% label for the destination is the same as the cross-reference label,
% |CTANlion|.
Show me
\hlstart{}{hlight=O,colormodel=named,color=OliveGreen}{CTANlion}
  the CTAN lion\hlend.

% Let's now point somewhere outside our document.  Eplain homepage is
% a good target.  In the similar spirit, let's consider two
% approaches.  The easy one is to use the |\url| command from
% |url.sty|.  Remember that we have customized the color of |url|
% hyperlinks, so this one will show up with a different color than the
% default dark-red.
Take me to \url{https://tug.org/eplain}.

% The second approach is to create an explicit URL link.  We specify
% yet another border highlighting mode, |P|, which means that the
% region underneath the bounding box of the link will be drawn inset
% into the page.  Also, let's set the color of the hyperlink to an RGB
% color |0.4,0.1,0.4|.  Since we cannot use commas to separate the
% color elements inside the options parameter to |\hlstart| (commas
% there separate options), we have to first create a user-defined
% color with |\definecolor| from |color.sty|, and use that in
% |\hlstart|.
\definecolor{mycolor}{rgb}{0.4,0.1,0.4}

Take me to
\hlstart{url}{hlight=P,colormodel=,color=mycolor}{https://tug.org/eplain}
  Eplain homepage\hlend.

\bye

8.2 Highlighting TeX comments in listings (lscommnt.tex) ¶

% (This file is public domain.)
% Demonstrate how Eplain can be used to include a TeX source file
% verbatim, typesetting comments in colored italic typewriter type.

% Load Eplain and LaTeX's color.sty package.
\input eplain
\beginpackages \usepackage{color} \endpackages
\nopagenumbers % Disable page numbers.
\font\commentfont = cmitt10 % Font for the comments (italic \tt).
% We'll define some `protected' macros with `@' in their names.
\makeatletter
% Define an equivalent of Eplain's \letreturn, to be able to assign
% various actions to the (active) percent character.
\begingroup \makeactive\%
  \gdef\letpercent{\let%}
\endgroup
% The listing hook to be called in \setuplistinghook, see below.  It
% makes `%' active and assigns it a `start comment' action.
\def\hlightcommentslisting{\makeactive\% \letpercent\start@comment}%
% This is what `%' is aliased to before a comment is started.
\def\start@comment{%
  \leavevmode % Needed in the very first line of the input to process
              % the new par (possibly inserting line number) before we
              % kick in with the color and stuff.
  \begingroup % Isolate color and font changes and the redefinitions.
    \commentfont
    \color[cmyk]{0.28,1,1,0.35}%
    \percentchar            % Produce the actual `%' and
    \letpercent\percentchar % make all following `%'s do the same.
    \letreturn\end@comment  % Call \end@comment at end-of-line.
}
% \end@comment (alias for ^^M inside a comment) will end the comment
% started by \start@comment.  We make ^^M active temporarily so that
% the active version of ^^M gets "frozen" into \end@comment.
\begingroup \makeactive\^^M % Avoid ^^M's from here on.
  \gdef\end@comment{\endgroup ^^M}%
\endgroup
\resetatcatcode % Make `@' again inaccessible for use in macro names.

% Define \setuplistinghook to setup comments highlighting, line
% numbering and omitting the last (empty) line of input.
\def\setuplistinghook{\hlightcommentslisting \linenumberedlisting
                      \nolastlinelisting}
% Isn't this fun?  This file typesets itself, with the extra bonus of
% the pretty-printed comments and numbered source lines!
\listing{lscommnt}
\bye