impatient-cn (13nov14)

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+% -*- coding: utf-8 -*-
+% This is part of the book TeX for the Impatient.
+% Copyright (C) 2003 Paul W. Abrahams, Kathryn A. Hargreaves, Karl Berry.
+% See file fdl.tex for copying conditions.
+
+\input macros
+%\chapter{A compendium \linebreak of useful macros}
+\chapter{实用宏集概述}
+
+\chapterdef{eplain}
+
+%This section describes |eplain.tex|, a collection of macros and other
+%definitions that extend  plain \TeX.
+%\bix^^|eplain.tex|
+%The descriptions of the various macros explain their purposes,
+%but usually do not explain how they
+%work or provide explicit details on
+%how to use them.  That information is contained in the source files for
+%|eplain.tex| and in the documentation that comes with it.
+%See \headcit{Resources}{resources} for how to obtain |eplain.tex|.
+这一章描述 |eplain.tex|，一组用于扩展 plain \TeX\ 的宏和其他定义。
+\bix^^|eplain.tex|
+对各种宏的描述解释了它们的目的，但通常不解释它们的工作原理或给出如何使用它们的详情。
+那些信息包含在 |eplain.tex| 源码文件以及它的文档中。
+在\headcit{资源}{resources}中介绍了获取 |eplain.tex| 的方法。
+
+
+%\section{Preliminaries}
+\section{预备定义}
+
+%We start with some macros for changing category codes and convenient
+%definitions for two of the commonly used ones.
+%^^{category codes//useful definitions for}
+我们先介绍几个修改类别码及定义两种常用类别码的宏。
+^^{类别码//有用的类别码定义}
+
+%\pix^^|\makeactive|
+%\pix^^|\letter|
+%\pix^^|\other|
+%\pix^^|\uncatcodespecials|
+\pix^^|\makeactive|
+\pix^^|\letter|
+\pix^^|\other|
+\pix^^|\uncatcodespecials|
+
+\csdisplay
+\def\makeactive#1{\catcode`#1 = \active \ignorespaces}%
+\chardef\letter = 11 \chardef\other = 12
+\def\uncatcodespecials{%
+   \def\do##1{\catcode`##1 = \other}%
+   \dospecials}% Defined in plain.
+|
+
+%In order to define `|^^M|' as an active character, you need to encase
+%the definition in a group and invoke some extra machinery.
+%\ttidxref{^^M}
+%The \pix^|\letreturn| macro lets you define `|^^M|' without that
+%extra machinery (which you can see in the definition below).
+为了将 `|^^M|' 定义为活动字符，你需要将定义放在编组中，并借助某些额外机制。
+\ttidxref{^^M}
+\pix^|\letreturn| 宏让你无需额外机制就可以定义 `|^^M|'（它的定义在下面可以看到）。
+
+\csdisplay
+{\makeactive\^^M \long\gdef\letreturn#1{\let^^M = #1}}%
+|
+
+%These macros consume one, two, or three arguments.
+这些宏用于吸收一个、两个或三个参量。
+
+%\pix^^|\gobble|
+%\pix^^|\gobbletwo|
+%\pix^^|\gobblethree|
+%\csdisplay
+%\def\gobble#1{}\def\gobbletwo#1#2{}%
+%\def\gobblethree#1#2#3{}%
+%|
+\pix^^|\gobble|
+\pix^^|\gobbletwo|
+\pix^^|\gobblethree|
+\csdisplay
+\def\gobble#1{}\def\gobbletwo#1#2{}%
+\def\gobblethree#1#2#3{}%
+|
+
+%Now we establish some conventions for reading the rest of the file.
+%Within the file we allow ``private'' control sequences that contain
+%`|@|' in their names.
+%These control sequences aren't accessible outside of this file (unless
+%you change the category code of `|@|' again).
+%\xrdef{eplainconv}
+现在我们建立一些惯例，用于读取这个文件的其他部分。
+在这个文件中我们允许名称带有 `|@|' 的``私有''控制序列。
+这些控制序列在这个文件之外无法使用（除非你再次修改 `|@|' 的类别码）。
+\xrdef{eplainconv}
+
+\csdisplay
+\catcode`@ = \letter    % Allow control sequences with @.
+\let\@plainwlog = \wlog % Don't log register allocations.
+\let\wlog = \gobble
+\newlinechar = `^^J
+|
+
+%The next two macros provide convenient forms of diagnostic output.
+%\pix^|\loggingall|  turns on all tracing, but causes the trace
+%output to appear only in the log file and not at your terminal.
+%\pix^|\tracingboxes| causes boxes to be displayed completely when
+%they're traced.  (\TeX\ normally shows only three levels of boxing
+%and five items within each box.)
+接下来两个宏提供了诊断输出的便捷方式。
+\pix^|\loggingall| 打开所有追踪，
+但让追踪输出仅写在日志文件中，而不在终端上显示。
+\pix^|\tracingboxes| 使得盒子被追踪时其内容完全显示。%
+（\TeX\ 通常只显示嵌套到三层的盒子，每个盒子显示五个项目。）
+
+\csdisplay
+\def\loggingall{\tracingcommands\tw@\tracingstats\tw@
+   \tracingpages\@ne\tracingoutput\@ne
+   \tracinglostchars\@ne\tracingmacros\tw@
+   \tracingparagraphs\@ne\tracingrestores\@ne
+   \showboxbreadth\maxdimen\showboxdepth\maxdimen}%
+\def\tracingboxes{\showboxbreadth = \maxdimen
+   \showboxdepth = \maxdimen}%
+|
+
+%The default thickness of rules is $0.4$\pt.
+%You can produce rules of any default thickness you choose by redefining
+%|\vruledefaultwidth|, |\hruledefaultheight|, and |\hruledefaultdepth|
+%and then using
+%\pix^^|\ehrule|
+%|\eh!-rule|
+%and \pix^|\evrule|
+%instead of |\hrule| and |\vrule|. (The `e'  stands for
+%``eplain''.)
+%^^{rules//thickness of}
+%If you give an explicit
+%dimension (e.g., |\ehrule height 16pt|), \TeX\ will use it.
+标线的默认厚度为 $0.4$\pt 。
+要生成默认厚度为任何值的标线，
+你可以重新定义 |\vruledefaultwidth|、|\hruledefaultheight| 及 |\hrule!-defaultdepth|，
+并将 |\hrule| 和 |\vrule| 换为 \pix^|\ehrule| 和 \pix^|\evrule|。%
+（名称中的 `e' 代表 ``eplain''。）%
+^^{标线//标线的厚度}
+如果你显式给出尺寸（比如 |\ehrule height 16pt|），\TeX\ 将使用它。
+
+\csdisplay
+\newdimen\hruledefaultheight  \hruledefaultheight = 0.4pt
+\newdimen\hruledefaultdepth   \hruledefaultdepth = 0.0pt
+\newdimen\vruledefaultwidth   \vruledefaultwidth = 0.4pt
+\def\ehrule{\hrule height\hruledefaultheight
+   depth\hruledefaultdepth}%
+\def\evrule{\vrule width\vruledefaultwidth}%
+|
+
+%The |\%| convention for writing a `|%|' character doesn't work when you
+%want to include that character in the token list of |\write|.
+%^^|\write//writing \b\tt\%\e\ with|
+%^^|%//writing out|
+%You can use ^|\percentchar| to achieve this.
+%We also redefine ^|^^L| to be nonouter so that you can use it in a
+%macro definition or argument.
+|\%| 通常用于写出 `|%|' 字符，但它无法包含在 |\write| 的记号列中。
+^^|\write//用它写出 \b\tt\%\e\|
+%^^|%//写出该字符|
+你可以用 ^|\percentchar| 达到此目的。
+我们还将 ^|^^L| 重新定义为非外部宏，因此你可以在宏定义或者参量中使用它。
+
+\csdisplay
+{\catcode`\% = \other \gdef\percentchar{%}}%
+ \def^^L{\par
+}%
+|
+
+%\pix^|\tokstostring| converts its argument into a list of ^{character
+%tokens}.
+%It uses only expansions that are handled in \TeX's gullet.
+%This property is necessary
+%for it to work with |\edef|.  It is used by the cross-referencing
+%macros (\xref{xrefs}).
+\pix^|\tokstostring| 将它的参量转换为一列^{字符记号}。
+它仅用到在 \TeX\ 的食道中处理的展开。
+此特性是和 |\edef| 一起使用时所必需的。它用于宏的交叉引用（\xref{xrefs}）。
+
+%In order to split the argument up at spaces, we have to use two
+%subsidiary macros. |\@ttsA| finds the spaces, and |\@ttsB| handles a
+%token sequence without any spaces.  Each space is replaced by the
+%expansion of \pix^|\spacesub|.
+为了将参量在空格处分开，我们要用到两个子宏。
+|\@ttsA| 用于寻找空格，而 |\@ttsB| 用于处理不带空格的控制序列。
+每个空格都被替换为 \pix^|\spacesub| 的展开。
+
+\csdisplay
+\def\tokstostring#1{\@ttsA#1 \ttsmarkA}%
+\def\@ttsA#1 #2\ttsmarkA{\ifempty{#1}\else
+      \@ttsB #1\@ttsmarkB
+      \ifempty{#2}\else
+         \spacesub\@ttsA#2\ttsmarkA\fi\fi}%
+\def\@ttsB#1{\ifx #1\@ttsmarkB\else
+      \string #1%
+      \expandafter\@ttsB\fi}%
+\def\@ttsmarkB{\@ttsmarkB}% should never be expanded
+\def\spacesub{+}%
+|
+
+%\noindent
+%\pix^|\ifempty| tests if its argument is empty.
+\noindent
+\pix^|\ifempty| 检测它的参量是否为空。
+
+\csdisplay
+\def\ifempty#1{\@ifempty #1\@emptymarkA\@emptymarkB}%
+\def\@ifempty#1#2\@emptymarkB{\ifx #1\@emptymarkA}%
+\def\@emptymarkA{\@emptymarkA}%
+|
+
+%The ^|\for|  macro implements a \TeX\ version of the ^{for loop} in
+%traditional programming languages.  These macros come directly from
+%\LaTeX.
+^|\for| 宏实现了其他传统编程语言中的^{for循环}的 \TeX\ 版本。
+这些宏直接取自 \LaTeX 。
+
+\csdisplay
+\def\for#1:=#2\do#3{\edef\@fortmp{#2}%
+   \ifx\@fortmp\empty \else
+      \expandafter\@forloop#2,\@nil,\@nil\@@#1{#3}\fi}%
+\def\@nnil{\@nil}%
+\def\@fornoop#1\@@#2#3{}%
+\def\@forloop#1,#2,#3\@@#4#5{\def#4{#1}\ifx #4\@nnil
+   \else #5\def#4{#2} ifx #4\@nnil \else
+      #5\@iforloop #3\@@#4{#5}\fi\fi}%
+\def\@iforloop#1,#2\@@#3#4{\def#3{#1}\ifx #3\@nnil
+   \let\@nextwhile=\@fornoop \else #4\relax
+      \let\@nextwhile=\@iforloop\fi
+         \@nextwhile#2\@@#3{#4}}%
+|
+
+%\pix^|\obeywhitespace| is useful for reproducing line breaks, blank lines,
+%and spaces in your input. It combines the effects of |\obey!-lines| and
+%|\obey!-spaces|,
+%^^|\obeylines| ^^|\obeyspaces|
+%and also causes spaces at the start of a line to
+%be printed. Tab characters are not affected by this; they still produce
+%normal glue.
+%\xrdef{ewhitesp}
+%^^{space characters}
+%^^|\fontdimen|
+%^^|\font|
+%^^|\letreturn|
+\pix^|\obeywhitespace| 用于重新生成你的输入中的换行、空行和空格。
+它结合了|\obey!-lines| 和 |\obey!-spaces| 的效果，
+^^|\obeylines| ^^|\obeyspaces|
+并把行首的空格也打印出来。制表符不受它影响；它们仍然生成正常的粘连。
+\xrdef{ewhitesp}
+^^{空格符}
+^^|\fontdimen|
+^^|\font|
+^^|\letreturn|
+
+\csdisplay
+\def\alwaysspace{\hglue\fontdimen2\the\font \relax}%
+{\makeactive\^^M \makeactive\ %
+\gdef\obeywhitespace{%
+\makeactive\^^M\def^^M{\par\indent}%
+\aftergroup\@removebox% Kill extra paragraph at end.
+\makeactive\ \let =\alwaysspace}}%
+\def\@removebox{\setbox0=\lastbox}
+|
+
+%\pix^|\frac| is a good way to print fractions in text when you don't
+%want to use |\over| and
+%``1/2'' just doesn't look right.  This macro is the answer to
+%\knuth{Exercise~11.6}.^^{fractions//slashed form}
+当你不想用 |\over|，而 ``1/2'' 看来又不够好时，
+你最好用 \pix^|\frac| 在正文中打印分式。
+这个宏是\knuth{练习~11.6}的答案。^^{分式//斜线形式}
+
+\csdisplay
+\def\frac#1/#2{\leavevmode
+   \kern.1em \raise .5ex \hbox{\the\scriptfont0 #1}%
+   \kern-.1em $/$%
+   \kern-.15em \lower .25ex \hbox{\the\scriptfont0 #2}}%
+|
+
+%The following macros produce logos that are useful in the \TeX\ world.
+%The \AMSTeX\ logo is from \knuth{page~420}.  The \LaTeX\ logo is
+%slightly modified from the one in |latex.tex| (we use a different font
+%for the `A'); similarly, the \BibTeX\ logo uses |\sevenrm| instead of a
+%true caps-and-small-caps font. The |.mf| source file for the \MF\ logo
+%is given in the \MF\ manual:
+%\smallskip
+%{\narrower\noindent
+%^{Knuth, Donald E.},{\sl The {\logosl METAFONT}book}.
+%  Reading, Mass.: Addison-Wesley, 1986.\par}
+%\smallskip
+%\pix^^|\LaTeX|
+%\pix^^|\AMSTeX|
+%\pix^^|\BibTeX|
+%\pix^^|\MF|
+下面这些宏生成 \TeX\ 圈中常用的标识。
+\AMSTeX\ 标识取自 \knuth{第~420~页}。
+\LaTeX\ 标识取自 |latex.tex| 并稍作修改（我们修改了 `A' 的字体）；
+类似地，\BibTeX\ 标识用 |\sevenrm| 字体代替真正的小型大写字体。
+\MF\ 标识的 |.mf| 源文件出现在 \MF\ 手册中：
+\smallskip
+{\narrower\noindent
+^{Knuth, Donald E.},{\sl The {\logosl METAFONT}book}.
+  Reading, Mass.: Addison-Wesley, 1986.\par}
+\smallskip
+\pix^^|\LaTeX|
+\pix^^|\AMSTeX|
+\pix^^|\BibTeX|
+\pix^^|\MF|
+
+\csdisplay
+\def\LaTeX{L\kern-.26em \raise.6ex\hbox{\fiverm A}%
+   \kern-.15em TeX}%
+\def\AMSTeX{$\cal A\kern-.1667em \lower.5ex\hbox{$\cal M$}%
+   \kern-.125em S$-\TeX}%
+\def\BibTeX{{\rm B\kern-.05em {\sevenrm I\kern-.025em B}%
+   \kern-.08em T\kern-.1667em \lower.7ex\hbox{E}%
+   \kern-.125emX}}%
+\font\mflogo = logo10
+\def\MF{{\mflogo META}{\tenrm \-}{\mflogo FONT}}%
+|
+
+%The next two macros produce boxes.  \pix^|\blackbox| produces a ``square
+%bullet'', used in the list macros (\xref{listmacs}).
+%\pix^|\makeblankbox| (from \knuth{page~311}) produces an unfilled
+%rectangle, with the thickness of the border rules given by the
+%arguments.
+接下来的两个宏用于生成盒子。
+\pix^|\blackbox| 生成一个方块，它用在列表宏中（\xref{listmacs}）。
+\pix^|\makeblankbox|（取自\knuth{第~311~页}）生成一个空心矩形，
+它的边框厚度由参量指定。
+
+\csdisplay
+\def\blackbox{\vrule height .8ex width .6ex depth -.2ex}%
+\def\makeblankbox#1#2{%
+   \hbox{\lower\dp0\vbox{\hidehrule{#1}{#2}%
+      \kern -#1% overlap rules
+      \hbox to \wd0{\hidevrule{#1}{#2}%
+         \raise\ht0\vbox to #1{}% vrule height
+         \lower\dp0\vtop to #1{}% vrule depth
+         \hfil\hidevrule{#2}{#1}}%
+      \kern-#1\hidehrule{#2}{#1}}}}%
+\def\hidehrule#1#2{\kern-#1\hrule height#1 depth#2
+   \kern-#2}%
+\def\hidevrule#1#2{\kern-#1{\dimen0 = #1
+   \advance\dimen0 by #2 \vrule width\dimen0}\kern-#2}%
+|
+
+%\pix^|\numbername| produces the written-out form of a number.  (If the
+%number is greater than ten, the macro just reproduces the numerals
+%of its argument.)
+\pix^|\numbername| 从数字生成其书面名称%
+（若数字大于十，这个宏仅重新生成参量的数值。）
+
+\csdisplay
+\def\numbername#1{\ifcase#1%
+   zero\or one\or two\or three\or four\or five%
+   \or six\or seven\or eight\or nine\or ten\or #1\fi}%
+|
+
+%\pix^|\testfileexistence| determines whether a file |\jobname.#1| is
+%non\-empty and sets |\if!-fileexists| appropriately.
+%^^{files//testing for existence of}
+%The file name in the argument need not end in a space token since
+%the macro provides the space token.
+\pix^|\testfileexistence| 判定文件 |\jobname.#1| 是否非空，
+并相应地设定 |\if!-fileexists| 的值。
+^^{文件//检测文件是否存在}
+参量中的文件名无需以空格记号结尾，因为这个宏已经提供了空格记号。
+
+\csdisplay
+\newif\iffileexists
+\def\testfileexistence#1{\begingroup
+      \immediate\openin0 = \jobname.#1\space
+      \ifeof 0\global\fileexistsfalse
+      \else \global\fileexiststrue\fi
+      \immediate\closein0
+   \endgroup}%
+|
+
+
+%\section Displays
+\section 陈列公式
+
+%By default, \TeX\ centers displayed material (the material between
+%|$$|'s).  ^^{displays, formatting} \pix^|\leftdisplays| causes displays
+%to be left-justified by default.  You can return to centered displays
+%with \pix^|\centereddisplays|.
+\TeX\ 默认将陈列公式（放在 |$$| 之间的公式）居中显示。
+^^{陈列公式//显示形式}\pix^|\leftdisplays| 让陈列公式默认为左对齐显示。
+你也可以用 \pix^|\centereddisplays| 命令恢复居中显示。
+
+%The macros here are more general than they need to be just for doing
+%left-justified displays.  For every display,
+%\pix^|\ifeqno| will be true if an |\eqno| occurred in the display.
+%\pix^|\ifleqno| will be true if an |\leqno| occurred.  If either
+%kind of equation number occurred, \pix^|\eqn| produces the text of the
+%equation number.  \pix^|\eq|  always produces the text of the
+%equation itself.
+这些宏提供让陈列公式左对齐显示之外的更多功能。
+对每个陈列公式，若 |\eqno| 存在则 \pix^|\ifeqno| 为真，
+若 |\leqno| 存在则 \pix^|\ifleqno| 为真。
+如果其中一种公式编号存在，\pix^|\eqn| 将排印公式编号的内容。
+而 \pix^|\eq| 始终排印公式本身的内容。
+
+%These macros are based on the code on \knuth{page~376}.
+这些宏基于\knuth{第~376~页}中的代码。
+
+\csdisplay
+\newif\ifeqno \newif\ifleqno
+\newtoks\@eqtoks \newtoks\@eqnotoks
+\def\eq{\the\@eqtoks}\def\eqn{\the\@eqnotoks}%
+\def\displaysetup#1$${%
+   \@displaytest#1\eqno\eqno\@displaytest}%
+\def\@displaytest#1\eqno#2\eqno#3\@displaytest{%
+   \if !#3!% No \eqno, check for \leqno:
+      \@ldisplaytest#1\leqno\leqno\@ldisplaytest
+   \else
+      \eqnotrue \leqnofalse % Have \eqno, not \leqno.
+      \@eqnotoks = {#2}\@eqtoks = {#1}%
+   \fi
+   \generaldisplay$$}%
+\def\@ldisplaytest#1\leqno#2\leqno#3\@ldisplaytest{%
+   \@eqtoks = {#1}%
+   \if !#3!%
+      \eqnofalse % No \leqno; we're done.
+   \else
+      \eqnotrue \leqnotrue % Have \leqno.
+      \@eqnotoks = {#2}%
+   \fi}%
+|
+
+%You can format displays differently by defining your own macro,
+%analogous to \pix^|\leftdisplays|.
+%The macro definition must
+%place a call on |\display!-setup| in |\every!-display| ^^|\everydisplay|
+%so as to ensure
+%that |\dis!-play!-setup| ^^|\displaysetup| is called at the start of
+%every display. The macro definition must also include a definition of
+%^|\generaldisplay|.
+通过自己定义类似 \pix^|\leftdisplays| 的宏，你可以给陈列公式设定不同的形式。
+这个宏定义中必须在 |\every!-display| ^^|\everydisplay|中调用 |\display!-setup|，
+以确保 |\dis!-play!-setup| ^^|\displaysetup|在每个陈列公式开始都被调用。
+这个宏定义必须也包含 ^|\generaldisplay| 的定义。
+
+\csdisplay
+\newtoks\previouseverydisplay
+\def\leftdisplays{%
+   \previouseverydisplay = \everydisplay
+   \everydisplay =
+     {\the\previouseverydisplay \displaysetup}%
+   \def\generaldisplay{%
+      \leftline{%
+         \strut \indent \hskip\leftskip
+         \dimen0 = \parindent
+         \advance\dimen0 by \leftskip
+         \advance\displaywidth by -\dimen0
+         \@redefinealignmentdisplays
+         \ifeqno \ifleqno
+            \kern-\dimen0
+            \rlap{$\displaystyle\eqn$}%
+            \kern\dimen0
+         \fi\fi
+         $\displaystyle{\eq}$%
+         \ifeqno \ifleqno\else
+            \hfill $\displaystyle{\eqn}$%
+         \fi\fi}}}%
+\def\centereddisplays{\let\displaysetup = \relax}%
+|
+
+%\filbreak
+%\pix^|\leftdisplays| must go to some pains to make sure that
+%|\dis!-play!-lines|, ^^|\displaylines|
+%^|\eqalignno|,  and ^|\leqalignno| still work properly.
+%|\eq| is typeset in math mode,
+%and |\halign| is illegal in math mode.
+%^^|\halign//illegal in math mode|
+%We use ^|\vcenter| to change the context so that  |\halign| becomes
+%legal again. We also remove the |\hfil| commands at the left of the template
+%to obtain the flush left formatting. Other than those changes, the macros
+%are the same as in |plain.tex|.
+%\filbreak
+\pix^|\leftdisplays| 必须历经辛苦地确保 |\dis!-play!-lines|^^|\displaylines|、
+^|\eqalignno| 和 ^|\leqalignno| 还能正常使用。
+|\eq| 在数学模式中排版，而 |\halign| 在数学模式中是不合法的。
+^^|\halign//在数学模式中不合法|
+我们用 ^|\vcenter| 改变环境，以让 |\halign| 重新变成合法的。
+我们还去掉模板左边的 |\hfil| 命令以得到左对齐格式。
+除这些改变之外，这些宏与 |plain.tex| 中的相同。
+
+\csdisplay
+\def\@redefinealignmentdisplays{%
+   \def\displaylines##1{\displ@y
+      \vcenter{\halign{\hbox to\displaywidth{$\@lign
+            \displaystyle####\hfil$}\crcr##1\crcr}}}%
+   \def\eqalignno##1{\displ@y
+      \vcenter{\halign to\displaywidth{%
+             $\@lign\displaystyle{####}$\tabskip\z@skip
+            &$\@lign\displaystyle{{}####}$
+               \hfil\tabskip\centering
+            &\llap{$\@lign####$}\tabskip\z@skip\crcr
+            ##1\crcr}}}%
+   \def\leqalignno##1{\displ@y
+      \vcenter{\halign to\displaywidth{%
+             $\@lign\displaystyle{####}$\tabskip\z@skip
+            &$\@lign\displaystyle{{}####}
+               $\hfil\tabskip\centering
+            &\kern-\displaywidth
+             \rlap{\kern-\parindent\kern-\leftskip$
+                \@lign####$}%
+             \tabskip\displaywidth\crcr
+            ##1\crcr}}}}%
+|
+
+
+%\section Time of day
+\section 日期时间
+
+%When \TeX\ starts up, it sets the values of the
+%^|\time|, ^|\day|, ^|\month|, and ^|\year| parameters.
+%^^{time of day}^^{date}
+%\pix^|\monthname|
+%produces the name of the month, abbreviated to three letters.
+%\pix^|\timestring| produces the current time, as in ``1:14\thinspace
+%p.m.''. \pix^|\timestamp| produces the text of the complete date, as
+%in ``23 Apr 1964\quad 1:14\thinspace p.m.''.
+\TeX\ 开始运行时设定了 ^|\time|、^|\day|、^|\month| 和 ^|\year| 参数的值。
+^^{日期时间}^^{日期}
+\pix^|\monthname| 排印月份名称的三字母缩写。
+\pix^|\timestring| 排印类似当前时间，类似于 ``1:14\thinspace p.m.''。
+\pix^|\timestamp| 排印完整日期文本，类似于 ``23 Apr 1964\quad 1:14\thinspace p.m.''。
+
+\csdisplay
+\def\monthname{%
+   \ifcase\month
+      \or Jan\or Feb\or Mar\or Apr\or May\or Jun%
+      \or Jul\or Aug\or Sep\or Oct\or Nov\or Dec%
+   \fi}%
+\def\timestring{\begingroup
+   \count0 = \time \divide\count0 by 60
+   \count2 = \count0   % The hour.
+   \count4 = \time \multiply\count0 by 60
+   \advance\count4 by -\count0   % The minute.
+   \ifnum\count4<10 \toks1 = {0}% Get a leading zero.
+   \else            \toks1 = {}%
+   \fi
+   \ifnum\count2<12 \toks0 = {a.m.}%
+   \else            \toks0 = {p.m.}%
+      \advance\count2 by -12
+   \fi
+   \ifnum\count2=0 \count2 = 12 \fi % Make midnight `12'.
+   \number\count2:\the\toks1 \number\count4
+   \thinspace \the\toks0
+\endgroup}%
+\def\timestamp{\number\day\space\monthname\space
+   \number\year\quad\timestring}%
+|
+
+
+%\section Lists
+\section 列表
+
+%\null
+%\xrdef{listmacs}
+%\bix^^{itemized lists}
+%\bix^^{enumerations}
+%\pix^|\numberedlist| produces numbered lists; |\endnumberedlist| ends
+%them.
+%\pix^|\unorderedlist| is analogous.
+%For either of these, items inside the lists begin
+%with \pix^|\li| (``list item'').  You can put \pix^|\listcompact|
+%at the beginning of a list if
+%you don't want any additional space between the items of that list.
+%Lists can be nested arbitrarily.
+\null
+\xrdef{listmacs}
+\bix^^{编号列表}
+\bix^^{列举}
+\pix^|\numberedlist| 排印编号列表；|\endnumberedlist| 结束该列表。
+类似地，\pix^|\unorderedlist| 排印无序列表。
+这两种列表中的项目以 \pix^|\li|（``list item''）开头。
+如果不需要列表项目间的额外间隔，你可以在列表开始处加上 \pix^|\listcompact|。
+列表可以任意嵌套。
+
+%You can control the spacing between the items more generally by
+%assigning values to the registers listed below.  If the items in
+%your lists tend to be long, you might want to make |\interitemskip|
+%nonzero.
+%The left indentation of each list item is given by |\par!-indent| plus
+%|\list!-left!-indent|; the right indentation of each list item
+%is given by |\list!-right!-indent|.
+更一般地，你可以通过改变下列寄存器的值控制列表项目间的间隔。
+如果列表项目经常比较长，你也许希望设定非零的 |\interitemskip|。
+各列表项目的左缩进量等于 |\par!-indent| 加 |\list!-left!-indent|；
+各列表项目的右缩进量等于 |\list!-right!-indent|。
+
+\csdisplay
+\newskip\abovelistskip  \abovelistskip = .5\baselineskip
+\newskip\interitemskip  \interitemskip = 0pt
+\newskip\belowlistskip  \belowlistskip = .5\baselineskip
+\newdimen\listleftindent  \listleftindent = \parindent
+\newdimen\listrightindent \listrightindent = 0pt
+\def\listcompact{\interitemskip = 0pt \relax}%
+|
+
+%Both numbered and unnumbered lists use the macros that follow.
+%We don't change |\parindent|, since many existing macros, e.g.,
+%|\footnote|, depend on |\parindent|.
+%We must account for the
+%possibility that items are more than one paragraph long. In this case, all
+%paragraphs after the first will be indented.
+%We use
+%|\leftskip| and |\rightskip| to indent the list items.
+%Indentation of displays is accounted for by changes to |\every!-dis!-play|.
+%^^|\everydisplay|
+编号列表和无序列表都使用下面这些宏。
+我们并不改变 |\parindent| 的值，
+因为很多现有的宏比如 |\footnote| 依赖于 |\parindent|。
+我们必须考虑到项目包含不止一个段落的可能性。
+在这种情形中，除第一个之外的所有段落都将被缩进。
+我们用 |\leftskip| 和 |\rightskip| 缩进列表项目。
+我们通过改变 |\every!-dis!-play| 设定陈列公式的缩进。
+^^|\everydisplay|
+
+\csdisplay
+\newdimen\@listindent
+\def\beginlist{%
+   \@listindent = \parindent
+   \advance\@listindent by \listleftindent
+   \everydisplay = \expandafter{\the\everydisplay
+      % Don't lose user's \everydisplay:
+      \advance\displayindent by \@listindent
+      \advance\displaywidth by -\@listindent
+      \advance\displaywidth by -\listrightindent}%
+   \nobreak\vskip\abovelistskip
+   \parskip = 0pt
+   % \leftskip shifts nested lists to the right on the page.
+   \advance\leftskip by \@listindent
+   \advance\rightskip by \listrightindent}%
+\def\printitem{\par\noindent
+   \llap{\hskip-\listleftindent \marker \enspace}}%
+\def\endlist{\vskip\belowlistskip}%
+|
+
+%\noindent
+%You can change the way the item labels are typeset by
+%redefining the |\numbered!-marker| macro.
+%\pix^^|\numberedmarker|
+\noindent
+通常重新定义 |\numbered!-marker| 宏，你可以改变项目标签的样式。
+\pix^^|\numberedmarker|
+
+\csdisplay
+\newcount\numberedlistdepth \newcount\itemnumber
+\newcount\itemletter
+\def\numberedmarker{%
+   \ifcase\numberedlistdepth
+       (impossible)%
+   \or \itemnumberout)%
+   \or \itemletterout)%
+   \else *%
+   \fi}%
+|
+
+%\noindent Here are the definitions of |\numberedlist| and
+%|\unorderedlist|.
+%Both definitions have the same structure.
+\noindent 这里是 |\numberedlist| 和 |\unorderedlist| 的定义。
+这两个定义的结构是相同的。
+
+\csdisplay
+\def\numberedlist{\environment{@numbered-list}%
+   \advance\numberedlistdepth by 1
+   \itemnumber = 1 \itemletter = `a
+   \beginlist \let\marker = \numberedmarker
+   \def\li{%
+      \ifnum\itemnumber=1\else \vskip\interitemskip \fi
+      \printitem
+      \advance\itemnumber by 1 \advance\itemletter by 1
+   }}%
+\def\itemnumberout{\number\itemnumber}%
+\def\itemletterout{\char\itemletter}%
+\def\endnumberedlist{\par
+   \endenvironment{@numbered-list}\endlist}%
+!bigskip
+\newcount\unorderedlistdepth
+\def\unorderedmarker{%
+   \ifcase\unorderedlistdepth
+       (impossible)%
+   \or \blackbox
+   \or ---%
+   \else *%
+   \fi}%
+\def\unorderedlist{\environment{@unordered-list}%
+   \advance\unorderedlistdepth by 1
+   \beginlist \itemnumber = 1
+   \let\marker = \unorderedmarker
+   \def\li{%
+      \ifnum\itemnumber=1\else \vskip\interitemskip \fi
+      \printitem \advance\itemnumber by 1
+   }}%
+\def\endunorderedlist{\par
+   \endenvironment{@unordered-list}\endlist}%
+|
+
+%\eix^^{itemized lists}
+%\eix^^{enumerations}
+\eix^^{编号列表}
+\eix^^{列举}
+
+
+%\section Verbatim listing
+\section 原文呈现
+
+%The \pix^|\listing| macro produces a verbatim listing of a specified
+%file in the |\tt| font.
+%It is based on the code on \knuth{page~380}.
+%Tabs produce a fixed amount of space, and form
+%feeds produce a page break.  Other control characters produce whatever
+%happens to be at that font position, which is generally not very useful.
+%By redefining |\setup!-listing!-hook|,
+%\pix^^|\setuplistinghook|
+%you can take additional actions that are appropriate for your particular
+%fonts and\slash or environment before the file is read in.
+\pix^|\listing| 宏用 |\tt| 字体排印指定文件的原文呈现。
+它基于\knuth{第~380~页}中的代码。
+制表符生成固定大小的间隔，而换页符生成一个分页。
+其他控制字符生成该字体所在位置的字符，这通常不太有用。
+通过重新定义 |\setup!-listing!-hook|\pix^^|\setuplistinghook|，
+你可以在读入文件前针对特别字体和\slash 或使用环境作额外定制。
+
+\csdisplay
+\def\listing#1{%
+   \par \begingroup \@setuplisting \setuplistinghook
+   \input #1 \endgroup}%
+\let\setuplistinghook = \empty
+\def\@setuplisting{%
+   \uncatcodespecials
+   \obeywhitespace \makeactive\` \makeactive\^^I
+   \def^^L{\vfill\eject}\tt}%
+{\makeactive\` \gdef`{\relax\lq}}% Defeat ligatures.
+{\makeactive\^^I\gdef^^I{\hskip8\fontdimen2\tt \relax}}%
+|
+
+
+%\section Tables of contents
+\section 目录
+
+%\null ^^{table of contents}
+%The macro \pix^|\writetocentry| writes a macro call to the file
+%|\jobname.toc|.  The first argument of |\writetocentry|, e.g.,
+%``chapter'', is used to compose the name of the called macro. The second
+%argument is the text to appear in the table of contents entry.
+%|\writetocentry| appends the page number to the macro call.  For
+%example:
+\null ^^{目录}
+\pix^|\writetocentry| 宏在 |\jobname.toc| 文件中写入一个宏调用。
+|\write!-tocentry| 的第一个参量，比如``chapter''，用于构成所调用的宏的名称。
+第二个参量是显示在目录项中的文本。
+|\writetocentry| 添加页码到宏调用中。例如：
+\csdisplay
+\writetocentry{chapter}{Introduction}
+|
+%\noindent
+%will produce the line:
+\noindent
+将生成下列这行：
+\csdisplay
+\tocchapterentry{Introduction}{2}
+|
+%\noindent in the |.toc| file, indicating
+%that `Introduction' started on page 2.
+\noindent 到 |.toc| 文件中，表示 `Introduction' 从第2页开始。
+
+%You can use |\writenumberedtocentry| to provide a third parameter, such
+%as a chapter number. For example:
+利用 |\writenumberedtocentry| 你还可以提供第三个参数，比如章编号。例如：
+\csdisplay
+\writenumberedtocentry{chapter}{The second chapter}{2}
+|
+%\noindent will write a line:
+\noindent 将写入这一行：
+\csdisplay
+\tocchapterentry{The second chapter}{2}{14}
+|
+%\noindent
+%You can also |\write| to |\tocfile| yourself.
+\noindent
+你也可以自己用 |\write| 命令写入 |\tocfile|。%
+\footnote{译注：原文的代码中遗漏 |\writetocentry| 的定义，
+而将 |\writenumberedtocentry| 的定义重复写了两遍，译文已修正此问题。
+实际上，在本书所用的 |eplain.tex| 1.9 中，
+|\writetocentry| 调用 |\writenumberedtocentry|，
+并在后者中根据第三个参数是否非空写入不同的目录项。
+但在 |\writenumberedtocentry| 的定义中有问题，
+导致写入 |tocfile| 文件的目录项始终有三个参数。
+在翻译时译者也同时修正 |eplain.tex| 中的这个错误。}
+
+\csdisplay
+\newwrite\tocfile \newif\iftocfileopened
+\def\opentocfile{\iftocfileopened\else
+      \tocfileopenedtrue
+      \immediate\openout\tocfile = \jobname.toc
+\fi}%
+\def\writetocentry#1#2{\ifrewritetocfile
+   \opentocfile
+   \write\tocfile{%
+      \expandafter\noexpand \csname toc#1entry\endcsname
+      {#2}{\folio}}%
+\ignorespaces\fi}%
+\def\writenumberedtocentry#1#2#3{\ifrewritetocfile
+   \opentocfile
+   \write\tocfile{%
+      \expandafter\noexpand \csname toc#1entry\endcsname
+      {#2}{#3}{\folio}}%
+\ignorespaces\fi}%
+|
+
+%To produce a table of contents, read the |.toc| file with
+%\pix^|\readtocfile|. You should call |\read!-tocfile| before the first
+%|\write!-toc!-entry|.  When you're processing the table of contents
+%without regenerating it, you should not rewrite
+%the |.toc| file---if you do, its contents will be lost.
+%The command
+%|\rewrite!-tocfile!-false| will prevent the rewrite.
+要排印出目录，只需用 \pix^|\readtocfile| 读取 |.toc| 文件。
+你应该在首次使用 |\write!-toc!-entry| 之前调用 |\read!-tocfile|。
+在处理目录且不想重新生成它时，
+务必不要改写 |.toc| 文件——如果你这样做，文件内容将会丢失。
+命令 |\rewrite!-tocfile!-false| 将禁止这种改写。
+
+\csdisplay
+\newif\ifrewritetocfile \rewritetocfiletrue
+\def\readtocfile{\testfileexistence{toc}%
+   \iffileexists
+      \input \jobname.toc
+      \ifrewritetocfile \opentocfile \fi
+   \fi}%
+|
+
+%Here are some definitions of possible |\toc|\dots|entry| macros.  These
+%definitions are meant only as examples---running leaders across the line
+%is usually not the best way to typeset a table of contents.
+这里给出可能出现的 |\toc|\dots|entry| 宏的一些定义。
+这些定义只是作为例子而已——在目录中使用指引线通常不是最好的做法。
+
+\csdisplay
+\def\tocchapterentry#1#2{\line{\bf #1 \dotfill\ #2}}%
+\def\tocsectionentry#1#2{%
+   \line{\quad\sl #1 \dotfill\ \rm #2}}%
+\def\tocsubsectionentry#1#2{%
+   \line{\qquad\rm #1 \dotfill\ #2}}%
+|
+
+
+%\section Cross-references
+\section 交叉引用
+
+%\null ^^{cross-references}
+%\xrdef{xrefs}
+%The macros that follow provide symbolic
+%cross-referencing, so that you can refer to something in another part of
+%a document by name instead of by its actual page number.
+%\pix^|\xrdef||{foo}| defines a label |foo| to be the current page
+%number, and \pix^|\xrefn||{foo}| produces that page number, e.g., $77$.
+%More often you'll want to say something like ``see p.\thinspace77'', so
+%\pix^|\xref||{foo}| produces ``p.\thinspace 77''.  If |foo| is not
+%defined, a warning message will be given.  |\xrefwarningfalse|
+%suppresses the warning.
+\null ^^{交叉引用}
+\xrdef{xrefs}
+接下来的宏提供了符号式的交叉引用，
+让你可以在文档中通过名称而非实际页码提及其它部分的东西。
+\pix^|\xrdef||{foo}| 定义标签 |foo| 为当前页码，
+而 \pix^|\xrefn||{foo}| 生成该页码，比如 $77$。
+你更常用的是类似 ``see p.\thinspace77'' 的写法，
+因此 \pix^|\xref||{foo}| 生成 ``p.\thinspace 77''。
+如果 |foo| 未定义，就会得到一个警告信息。
+|\xrefwarningfalse| 取消这种警告。
+
+%These macros provide no protection against duplicate definitions.  You can
+%check for duplicate definitions by sorting the cross-reference file and
+%checking, either mechanically or by eye, for adjacent definitions of
+%the same symbol.
+这些宏没有提供对重复定义的保护。要检查重复定义，
+你可以将交叉引用文件排序，并用工具或肉眼检查同一个符号的相邻定义。
+
+\csdisplay
+\newif\ifxrefwarning  \xrefwarningtrue
+\def\xrdef#1{\begingroup
+   \xrlabel{#1}%
+   \edef\@wr{\@writexrdef{\the\@xrlabeltoks}}%
+   \@wr
+   \endgroup \ignorespaces}%
+\def\@writexrdef#1{\write\reffile{%
+     \string\gdef
+        \expandafter\string\csname#1\endcsname
+        {\noexpand\folio}\percentchar}}%
+\def\xrefnumber#1{%
+   \xrlabel{#1}%
+   % \@xrlabeltoks now has the control sequence name.
+   \toks0 =
+      \expandafter{\csname\the\@xrlabeltoks\endcsname}%
+   \expandafter \ifx\the\toks0\relax
+      \ifxrefwarning \message{Undefined label
+         `\tokstostring{#1}'.}\fi
+      {\let\spacesub = \space
+       \expandafter\xdef\the\toks0
+          {`{\tt \tokstostring{#1}}'}}\fi
+   \the\toks0}%
+\def\xref#1{p.\thinspace\xrefnumber{#1}}%
+\def\xrefn#1{\xrefnumber{#1}}%
+|
+
+%This macro turns a label into a list of character tokens in the token
+%register |\labeltoks|.  A label can include blanks and control sequences in
+%it as well as normal characters, but it can't include braces.
+这个宏将一个标签转换为一列字符记号，并放在寄存器 |\labeltoks| 中。
+除了普通字符之外，标签还可以包含空格和控制序列，但不能包含花括号。
+
+\csdisplay
+\newtoks\@xrlabeltoks
+\def\xrlabel#1{\begingroup
+      \escapechar = `\_ \edef\tts{\tokstostring{#1_}}%
+      \global\@xrlabeltoks = \expandafter{\tts}%
+   \endgroup}%
+|
+
+%It takes two passes to get the cross-references right, since the
+%definitions are written out to the auxiliary file |\jobname.aux|.
+%\pix^|\readreffile| reads them back in. If you don't issue this command
+%before the first definition, you'll lose all the definitions from the
+%previous run.
+需要运行两遍才能得到正确的交叉引用，
+因为这些定义是写出到辅助文件|\jobname.aux| 里的。
+\pix^|\readreffile| 命令用于将它们读取回来。
+如果你不在第一个定义之前执行此命令，你将丢失上一遍运行得到的定义。
+
+\csdisplay
+\newwrite\reffile \newif\ifreffileopened
+\def\openreffile{\ifreffileopened\else
+      \reffileopenedtrue
+      \immediate\openout\reffile = \jobname.aux
+   \fi}%
+\def\readreffile{%
+   \testfileexistence{aux}%
+   \iffileexists
+      \begingroup
+         \@setletters
+         \input \jobname.aux
+      \endgroup
+   \else
+      \message{No cross-reference file; I won't give you
+         warnings about undefined labels.}%
+      \xrefwarningfalse
+   \fi
+   \openreffile}%
+\def\@setletters{%
+   \catcode`_ = \letter \catcode`+ = \letter
+   \catcode`- = \letter \catcode`@ = \letter
+   \catcode`0 = \letter \catcode`1 = \letter
+   \catcode`2 = \letter \catcode`3 = \letter
+   \catcode`4 = \letter \catcode`5 = \letter
+   \catcode`6 = \letter \catcode`7 = \letter
+   \catcode`8 = \letter \catcode`9 = \letter
+   \catcode`( = \letter \catcode`) = \letter}%
+|
+
+%^^{equations, labelling}
+%You can give symbolic names to equations in a similar way, using
+%\pix^|\eqdef| and \pix^|\eqref|.  |\eqdef| inserts its own |\eqno|
+%command, so it must be invoked in a place where |\eqno| is legal.
+^^{公式编号}
+按同样的方式，你可以用 \pix^|\eqdef| 和 \pix^|\eqref| 给出公式的符号式名称。
+|\eqdef| 插入自己的 |\eqno| 命令，因此它必须用在能用 |\eqno| 的地方。
+
+\csdisplay
+\newcount\eqnumber
+\def\eqdef#1{\global\advance\eqnumber by 1
+   \expandafter\xdef
+      \csname#1eqref\endcsname{\the\eqnumber}%
+   \immediate\write\reffile{\string\def
+      \expandafter\string\csname#1eqref\endcsname
+         {\the\eqnumber}}%
+   \eqno
+   \eqprint{\the\eqnumber}}%
+|
+
+%|\eqref| produces ``(equation number)''.  You can handle fancier
+%formatting by redefining \pix^|\eqprint|. For example, you could redefine
+%it so that the equation numbers include
+%the chapter number.
+|\eqref| 生成``(公式编号)''。
+通过重新定义 \pix^|\eqprint|，你可以实现更加复杂的格式。
+举个例子，你可以重新定义它，使得公式编号中包含章编号。
+
+\csdisplay
+\def\eqref#1{%
+   \expandafter \ifx \csname#1eqref\endcsname \relax
+      \ifxrefwarning \message{Undefined equation label
+         `#1'.}\fi
+      \expandafter\def\csname#1eqref\endcsname{00}%
+   \else \eqprint{\csname#1eqref\endcsname}%
+   \fi}%
+\def\eqprint#1{(#1)}%
+|
+
+
+%\section Environments
+\section 环境
+
+%\null ^^{environments} These macros let you define your own named groups
+%(environments) for parts of your manuscript.  Like \TeX\ groups, these
+%groups can be nested, and in fact their nesting can be intertwined with
+%the nesting of \TeX\ groups.  If the names at the beginning and end of
+%an environment don't match, you'll get an error message. The macros are
+%designed so that the message you get when such an error occurs will give
+%you a good chance of localizing the cause of the error easily.
+\null ^^{环境}这些宏让你可以将手稿的一部分定义为命名编组（环境）。
+类似 \TeX\ 的编组，这些编组也可以嵌套，
+而且实际上它们的嵌套可以与 \TeX\ 编组的嵌套相互交织。
+如果开始的名称和结束的名称不匹配，你将得到一个错误信息。
+这些宏如此设计，使得当你得到这种错误信息时，你有机会方便地定位错误的来源。
+
+%You begin an environment with
+%\pix^^|\environment|
+%|\envi!-ron!-ment| |{foo}| and end it with |\endenvironment{foo}|, where
+%|foo| is the name of the environment.  Our macros slightly improve on
+%the answer to Exercise~5.7 of \texbook, by doing some checks on
+%|\begingroup| and |\endgroup| pairs, as well as making sure
+%|\environment| and |\endenvironment| pairs match.
+用 \pix^^|\environment||\envi!-ron!-ment| |{foo}| 开始一个环境，
+用 |\endenvironment{foo}| 结束该环境，其中 |foo| 是该环境的名称。
+我们的宏稍微改进了 \texbook\ 练习~5.7，
+添加对 |\begingroup| 和 |\endgroup| 是否配对的检查，
+并确保 |\environment| 和 |\endenvironment| 能够匹配。
+
+\csdisplay
+\def\environment#1{\ifx\@groupname\undefined\else
+      \errhelp = \@unnamedendgrouphelp
+      \errmessage{`\@groupname' was not closed by
+         \string\endenvironment}\fi
+   \def\@groupname{#1}%
+   \begingroup
+      \let\@groupname = \undefined \ignorespaces}%
+\def\endenvironment#1{\endgroup
+   \def\@thearg{#1}%
+   \ifx\@groupname\@thearg
+   \else
+      \ifx\@groupname\undefined
+         \errhelp = \@isolatedendenvironmenthelp
+         \errmessage{Isolated
+            \string\endenvironment\space for `#1'}%
+      \else
+         \errhelp = \@mismatchedenvironmenthelp
+         \errmessage{Environment `#1' ended,
+            but `\@groupname' started}%
+         \endgroup % Probably a typo in the names.
+      \fi
+   \fi
+   \let\@groupname = \undefined \ignorespaces}%
+|
+
+%We also define help messages for each of the errors above.
+%^^|\newhelp|
+你也可以给上述这些错误定义它们各自的帮助信息。
+^^|\newhelp|
+
+\csdisplay
+\newhelp\@unnamedendgrouphelp{%
+   Most likely, you just forgot an^^J%
+   \string\endenvironment.
+   Maybe you should try inserting another^^J%
+   \string\endgroup to recover.}%
+\newhelp\@isolatedendenvironmenthelp{%
+   You ended an environment X, but^^J%
+   no \string\environment\space to start it
+   is anywhere in sight.^^J%
+   You might also be at an
+   \string\endenvironment\space that would match^^J%
+   a \string\begingroup, i.e., you forgot an
+   \string\endgroup.}%
+\newhelp\@mismatchedenvironmenthelp{%
+   You started an environment X, but^^J%
+   you ended it with Y.  Maybe you made a typo
+   in one or the other^^J%
+   of the names.}%
+|
+
+%Some environments should not be allowed to occur within
+%another environment.  Let's call these environments
+%``outer environments''. |\check!-env| checks that no outer environment
+%is currently in effect and complains if one is. To use |\check!-env|, you
+%must issue the command |\environment!-true| at the beginning of every
+%outer environment.
+某些环境应当不允许出现在其他环境内部。我们称这些环境为``外部环境''。
+|\check!-env| 检测是否不存在当前有效的外部环境，若存在的话给出警告信息。
+要使用 |\checkenv|，你必须在每个外部环境开头处执行 |\environment!-true| 命令。
+
+\csdisplay
+\newif\ifenvironment
+\def\checkenv{%
+   \ifenvironment
+      \errhelp = \@interwovenenvhelp
+      \errmessage{Interwoven environments}%
+      \endgroup
+   \fi}%
+\newhelp\@interwovenenvhelp{%
+   Perhaps you forgot to end the previous^^J%
+   environment? I'm finishing off the current group,^^J%
+   hoping that will fix it.}%
+|
+
+
+%\section Justification
+\section 对齐
+
+%\bix^^{flush left}
+%\bix^^{flush right}
+%\bix^^{centered text}
+%The three macros \pix^|\flushleft|, \pix^|\flushright|, and
+%\xrdef{eplaincenter}
+%\pix^|\center| justify  the text on the following lines in the indicated way.
+%The command should appear on a line by itself.
+%Both the command and the text should be enclosed in a group---%
+%the end of the group indicates the end of the text.
+%The entire group is set as a single paragraph, with lines filled out
+%on one side or another as appropriate.  Blank lines are reproduced.
+\bix^^{居左对齐}
+\bix^^{居右对齐}
+\bix^^{居中文本}
+这三个宏 \pix^|\flushleft|、\pix^|\flushright| 和
+\xrdef{eplaincenter}\pix^|\center| 将后面各行文本以指定方式对齐。
+这种命令必须单独写在一行中。
+命令和文本应该包含在一个编组中——编组的结束就表示文本的结束。
+整个编组作为一个段落排版，各行视情况在一边或两边用空白填充。
+空行依原样显示。
+
+\csdisplay
+\begingroup
+   \catcode `\^^M = \active
+   \globaldefs = 1 %
+   \def\flushleft{\beforejustify %
+      \aftergroup\@endflushleft %
+      \def^^M{\null\hfil\break}%
+      \def\@eateol^^M{}\@eateol}%
+   \def\flushright{\beforejustify %
+      \aftergroup\@endflushright %
+      \def^^M{\break\null\hfil}%
+      \def\@eateol^^M{\hfil\null}\@eateol}%
+   \def\center {\beforejustify %
+      \aftergroup\@endcenter %
+      \def^^M{\hfil\break\null\hfil}%
+      \def\@eateol^^M{\hfil\null}\@eateol}%
+\endgroup
+|
+
+%The following commands are called as a result of the |\after!-group|
+%^^|\aftergroup|
+%in the definitions of |\flush!-left|, |\flush!-right|, and |\center|.
+%They perform the necessary cleanup operations.
+在 |\flush!-left|、|\flush!-right| 和 |\center| 的定义中，
+下述这些命令出现在 ^|\aftergroup| 的后面；
+它们在编组结束后被调用，以执行一些必需的清理工作。
+
+\csdisplay
+\def\@endflushleft{\unpenalty
+   {\parfillskip = 0pt plus 1 fil\par}%
+   \ignorespaces}%
+\def\@endflushright{%
+   % Remove the \hfil\null\break we just put on.
+   \unskip \setbox0=\lastbox \unpenalty
+   % We have fil glue at the left of the line;
+   % \parfillskip shouldn't affect that.
+   {\parfillskip = 0pt \par}\ignorespaces}%
+\def\@endcenter{%
+   % Remove the \hfil\null\break we just put on.
+   \unskip \setbox0=\lastbox \unpenalty
+   % We have fil glue at the left of the line;
+   % \parfillskip must balance it.
+   {\parfillskip = 0pt plus 1fil \par}\ignorespaces}%
+\def\beforejustify{%
+   \par\noindent
+   \catcode`\^^M = \active
+   \checkenv \environmenttrue}%
+|
+
+%\eix^^{flush left}
+%\eix^^{flush right}
+%\eix^^{centered text}
+\eix^^{居左对齐}
+\eix^^{居右对齐}
+\eix^^{居中文本}
+
+
+%\section Tables
+\section 表格
+
+%The \pix^|\makecolumns| macro enables you to give all the entries in a
+%table without having to worry about where the columns break.  For
+%example, if you're typing a long alphabetical list that will be
+%formatted in several columns, you usually won't know in advance where
+%one column ends and the next begins.  Moreover, if another item gets
+%added, the column breaks will change.
+\pix^|\makecolumns| 宏允许你给出表格的所有元素，而无需担心如何分栏。
+例如，在输入一个冗长的按字母排序的名单，并分多栏显示时，
+你通常不知道在何处结束一栏开始下一栏。
+另外，如果添加了另一项，各栏的划分将会改变。
+
+%|\makecolumns| takes two (delimited) arguments: the total number of
+%entries in the table and the number of columns in the table.  For
+%example, `|\makecolumns 37/3:|' specifies a three-column table whose
+%entries are the next $37$ lines.  You can adjust the positioning of the
+%table on the page by changing |\parindent|, which determines the space
+%to the left, and |\hsize|, which determines the space from the left
+%margin of the page to the right of the block.  You can allow a page
+%break above the |\valign|
+%^^|\valign//used in {\tt\\makecolumns}|
+%by changing
+%\pix^|\abovecolumnspenalty|.
+|\makecolumns| 有两个（定界）参量：表格元素的总数以及表格的栏数。
+例如，`|\makecolumns 37/3:|' 指定一个三栏表格，其元素为接下来的 $37$ 行。
+要调整表格在页面上的位置，你可以修改 |\parindent| 以确定表格左侧的空白，
+以及 |\hsize| 以确定从页面左边缘到表格右侧的距离。
+要允许在 |\valign| ^^|\valign//用于 {\tt\\makecolumns}| 之前分页，
+你可以修改 \pix^|\abovecolumnspenalty|。
+
+\csdisplay
+\newcount\abovecolumnspenalty
+\abovecolumnspenalty = 10000
+\newcount\@linestogo      % Lines remaining to process.
+\newcount\@linestogoincolumn % Lines remaining in column.
+\newcount\@columndepth    % Number of lines in a column.
+\newdimen\@columnwidth    % Width of each column.
+\newtoks\crtok  \crtok = {\cr}%
+\def\makecolumns#1/#2: {\par \begingroup
+   \@columndepth = #1 \advance\@columndepth by #2
+   \advance\@columndepth by -1
+   \divide \@columndepth by #2
+   \@linestogoincolumn = \@columndepth \@linestogo = #1
+   \def\@endcolumnactions{%
+      \ifnum \@linestogo<2
+         \the\crtok \egroup \endgroup \par
+            % End \valign and \makecolumns.
+      \else
+         \global\advance\@linestogo by -1
+         \ifnum\@linestogoincolumn<2
+            \global\@linestogoincolumn = \@columndepth
+            \the\crtok
+         \else &\global\advance\@linestogoincolumn by -1
+         \fi
+      \fi}%
+   \makeactive\^^M\letreturn\@endcolumnactions
+   \@columnwidth = \hsize
+   \advance\@columnwidth by -\parindent
+   \divide\@columnwidth by #2
+   \penalty\abovecolumnspenalty
+   \noindent % It's not a paragraph (usually).
+   \valign\bgroup
+      &\hbox to \@columnwidth{\strut ##\hfil}\cr
+}% The next end-of-line starts everything going.
+|
+
+
+%\section Footnotes
+\section 脚注
+
+%\null ^^{footnotes}
+%Footnotes are most commonly typeset by using a raised number as the
+%reference mark.  We define the \pix^|\numberedfootnote| macro to do
+%this.  It also redefines ^|\vfootnote| to allow slightly more general
+%formatting of footnotes than \plainTeX\ does. The dimension register
+%\pix^^|\footnotemarkseparation| |\foot!-note!-mark!-sepa!-ra!-tion|
+%controls the space between the footnote mark (e.g., the number) and the
+%beginning of the text. The \pix^|\everyfootnote| tokens are inserted
+%before producing the footnote.
+\null ^^{脚注}
+脚注通常用升高的数字作为参考符号。
+我们定义的 \pix^|\numberedfootnote| 宏就是这样的。
+它还重新定义了 ^|\vfootnote|，以支持比 \plainTeX\ 更一般的脚注格式。
+尺寸寄存器 \pix^^|\footnotemarkseparation| |\foot!-note!-mark!-sepa!-ra!-tion|
+控制脚注符号（比如数字）与脚注文本之间的间隔。
+\pix^|\everyfootnote| 记号在每个脚注之前插入。
+
+%The \plainTeX\ definitions of |\footnote| and |\vfootnote| are
+%preserved in |\@plain!-footnote| and |\@plain!-vfootnote| in case you should
+%need them.
+\PlainTeX\ 中的 |\footnote| 和 |\vfootnote| 定义保留在
+|\@plain!-footnote| 和 |\@plain!-vfootnote| 中，以备不时之需。
+
+\csdisplay
+\newcount\footnotenumber \newtoks\everyfootnote
+\newdimen\footnotemarkseparation
+\footnotemarkseparation = .5em
+\let\@plainfootnote = \footnote
+\let\@plainvfootnote = \vfootnote
+\def\vfootnote#1{\insert\footins\bgroup
+  \interlinepenalty\interfootnotelinepenalty
+  \splittopskip\ht\strutbox \splitmaxdepth\dp\strutbox
+  \floatingpenalty\@MM
+  \leftskip\z@skip \rightskip\z@skip \spaceskip\z@skip
+  \xspaceskip\z@skip
+  \everypar = {}%
+  \the\everyfootnote
+  \indent\llap{#1\kern\footnotemarkseparation}\footstrut
+  \futurelet\next\fo@t}%
+\def\numberedfootnote{\global\advance\footnotenumber by 1
+  \@plainfootnote{$^{\number\footnotenumber}$}}%
+|
+
+
+%\section Double columns
+\section 双栏
+
+%\null ^^{double columns} The |\doublecolumns| command begins
+%double-column output, while the |\single!-column|
+%\pix^^|\singlecolumn|
+%command restores
+%single-column output.  You can switch back and forth between them
+%on a single page.
+%The glue specified by |\above!-double!-column!-skip| and
+%|\below!-double!-column!-skip| is inserted before and after the
+%double-column material.
+\null ^^{双栏} |\doublecolumns| 命令开始双栏输出，
+而 |\single!-column| \pix^^|\singlecolumn| 命令恢复单栏输出。
+两者可以在同个页面上相互切换。
+用 |\above!-double!-column!-skip| 和 |\below!-double!-column!-skip|
+指定的粘连插入到双栏素材的前面和后面。
+
+%The approach is derived from \knuth{page~417}.
+它们的实现方式来源于\knuth{第~417~页}。
+
+\csdisplay
+\newskip\abovedoublecolumnskip
+\newskip\belowdoublecolumnskip
+\abovedoublecolumnskip = \bigskipamount
+\belowdoublecolumnskip = \bigskipamount
+\newdimen\gutter            \gutter = 2pc
+\newdimen\doublecolumnhsize \doublecolumnhsize = \hsize
+\newbox\@partialpage        \newdimen\singlecolumnhsize
+\newdimen\singlecolumnvsize \newtoks\previousoutput
+\def\doublecolumns{\par % Don't start in horizontal mode.
+   \previousoutput = \expandafter{\the\output}
+   \advance\doublecolumnhsize by -\gutter
+   \divide\doublecolumnhsize by 2
+   \output = {\global\setbox\@partialpage =
+         \vbox{\unvbox255\vskip\abovedoublecolumnskip}}%
+   \pagegoal = \pagetotal \break % Expands \output above.
+   \output = {\doublecolumnoutput}%
+   \singlecolumnhsize = \hsize
+   \singlecolumnvsize = \vsize
+   \hsize = \doublecolumnhsize \vsize = 2\vsize}%
+|
+
+%The |\@double!-column!-split| macro does the actual splitting.
+%Insertions are assumed to be single-column material.  If you don't want
+%this to be the case, you'll have to modify the output routine.  After
+%|\@double!-column!-split| has done its work, |\box255| will have the
+%double-column material.  The double-column material will be preceded by
+%any single-column material that was typeset before |\doublecolumns| was
+%invoked. |\box4| will have the material that didn't fit on the page.
+|\@double!-column!-split| 宏执行实际的分栏。插入项被当作单栏素材；
+如果这不是你想要的，你需要修改输出例行程序。
+在 |\@double!-column!-split| 完成之后，|\box255| 将包含双栏素材。
+双栏素材前面是调用 |\doublecolumns| 前的单栏素材。
+|\box4| 将包含无法放入该页面的素材。
+
+\csdisplay
+\def\@doublecolumnsplit{%
+   \splittopskip = \topskip \splitmaxdepth = \maxdepth
+   \dimen0 = \singlecolumnvsize
+      \advance\dimen0 by -\ht\@partialpage
+      \advance\dimen0 by -\ht\footins
+      \advance\dimen0 by -\skip\footins
+      \advance\dimen0 by -\ht\topins
+   \begingroup
+      \vbadness = 10000
+      \global\setbox1=\vsplit255 to \dimen0 \wd1=\hsize
+      \global\setbox3=\vsplit255 to \dimen0 \wd3=\hsize
+   \endgroup
+   \global\setbox4=\vbox{\unvbox255
+      \penalty\outputpenalty}%
+   \global\setbox255=\vbox{\unvbox\@partialpage
+      \hbox to \singlecolumnhsize{\box1\hfil\box3}%
+      \vfill}}%
+|
+%\needspace{.5in}
+%|\double!-column!-out!-put| is the real output routine.  We call the old
+%|\output| to do the work of actually shipping out the box.
+\needspace{.5in}
+|\double!-column!-out!-put| 是真正的输出例行程序。
+我们调用 |\output| 执行实际的盒子送出工作。
+
+\csdisplay
+\def\doublecolumnoutput{\@doublecolumnsplit
+   \hsize = \singlecolumnhsize \vsize = \singlecolumnvsize
+   \previousoutput \unvbox4}%
+|
+
+%|\singlecolumn| resumes typesetting in one column.  It assumes that
+%|\doublecolumns| has been called.
+|\singlecolumn| 恢复单栏排版。它假定已调用了 |\doublecolumns|。
+
+\csdisplay
+\def\singlecolumn{\par % Don't start in horizontal mode.
+   \output = {\global\setbox1 =
+      \vbox{\unvbox255\vskip\abovedoublecolumnskip}}%
+   \pagegoal = \pagetotal \break \setbox255 = \box1
+   {\singlecolumnvsize = \ht255
+      \divide\singlecolumnvsize by 2
+      \advance\singlecolumnvsize by +\ht\@partialpage
+      \advance\singlecolumnvsize by +\ht\footins
+      \advance\singlecolumnvsize by +\skip\footins
+      \advance\singlecolumnvsize by +\ht\topins
+   \@doublecolumnsplit}%
+   \hsize = \singlecolumnhsize
+   \vsize = \singlecolumnvsize
+   \output = \expandafter{\the\previousoutput}%
+   \unvbox255}%
+|
+
+%\margin{`Sensible paragraph skips' section deleted}
+\margin{`Sensible paragraph skips' section deleted}
+
+
+%\section Finishing up
+\section 收尾
+
+%We now must undo the changes that we made when we started (see
+%\xref{eplainconv}).  We also give a version identification, which is
+%subsequently available in |\fmt!-name| and |\fmt!-version|.
+现在我们必须撤销开始的修改（见\xref{eplainconv}）。
+我们还给出版本标识，它可以在 |\fmt!-name| 和 |\fmt!-version| 中得到。
+
+\csdisplay
+\let\wlog = \@plainwlog \catcode`@ = \other
+\def\fmtname{eplain}%
+{\edef\plainversion{\fmtversion}%
+ \xdef\fmtversion{1.0: 15 May 1990
+   (and plain \plainversion)}%
+}%
+|
+
+%\eix^^|eplain.tex|
+\eix^^|eplain.tex|
+
+\ifoldeplain\else\ifcompletebook\else
+\vskip4em{\sectionfonts\leftline{本章索引}}
+\readindexfile{i}
+\fi\fi
+
+\endchapter
+\byebye