\chardef\escape=0
\chardef\open=1
\chardef\close=2
\chardef\letter=11
\chardef\other=12
%\chardef\active=13		 % is defined in Plain already

\chardef\uscode=\catcode`\_

\catcode`\_=\letter
%\font\tentex=cmtex10		% typewriter extended ASCII 10pt
%\let\ttex=\tentex		% only with base size 10pt
\def\ttex{\tt}			% as a substitute

\def\setup_verbatim{%
   \def\do##1{\catcode`##1\other}\dospecials
   \parskip\z@skip \parindent\z@
   \catcode`\`\active \@noligs
   \obeylines \@vobeyspaces \obeytabs \frenchspacing
   \ttex
   }

\let\tab=\space
\begingroup
   \catcode`\^^I=\active%	% Attention: no tabs!
   \gdef\obeytabs{\catcode`\^^I=\active\def^^I{\tab}}
   \global\let^^I=\tab%    % if an active tab appears in a \write
\endgroup
\let\origvert=|
\chardef\vbar=`\|

\def\makebaractive{\catcode`\|\active}
\def\makebarother{\catcode`\|\other}
\makebaractive

\def|{%
   \leavevmode
   \hbox\bgroup
      \let\par\space \setup_verbatim
      \let|\egroup
   }
\let\@@verbatim=\@verbatim
\def\@verbatim{%
   \makebarother
   \@@verbatim
   }
\let\@@tabarray=\@tabarray
\def\@tabarray{%
   \makebarother
   \@@tabarray
   }

\def\multicolumn#1{%
   \multispan{#1}%
   \begingroup
      \makebarother
      \restof_multicolumn
   }
\def\restof_multicolumn#1#2{%
      \@mkpream{#1}%
      \def\@sharp{#2}%
      \let\protect\relax
      \let\@startpbox\@@startpbox  \let\@endpbox\@@endpbox
      \@arstrut \@preamble
   \endgroup
   \ignorespaces
   }

\let\@@mkpream=\@mkpream
\def\@mkpream#1{%
   \@@mkpream{#1}%
   \makebaractive
   }
\begingroup
   \def\@makeother{\noexpand\@makeother\noexpand}
   \xdef\@sanitize{\@sanitize\@makeother\|}
\endgroup
\newif\if@print

\def\begin_verbatim{%
   \endgraf
   \bigbreak
   \begingroup
      \setup_verbatim \makebarother
      \@printtrue
      \ignore_rest_line
   }
\let\end_verbatim=\endgroup		% internal command !
\begingroup
   \obeylines%	% ^^M is active! ==> every line must end with %
   \gdef\ignore_rest_line#1^^M{\set_next_line}%
   \gdef\set_next_line#1^^M{\do_set{#1}}%
\endgroup

\def\do_set#1{%
   \endgraf
   \check_print{#1}%
   \if@print  \indent \print_char#1\end_line\end_line
   \else  \let\set_next_line\end_verbatim
   \fi
   \set_next_line
   }
\let\end_line=\relax
\begingroup
\obeyspaces\obeytabs
\gdef\check_print#1{\cut_at_tab#1^^I\end_line}
\gdef\cut_at_tab#1^^I#2\end_line{\check_first_part#1 \end_line}% blank !
\gdef\check_first_part#1 #2\end_line{\do_check{#1}}
\endgroup
\def\do_check#1{%
   \def\@line{#1}%
   \ifx \@line\end_verbatim_line  \@printfalse
   \fi
   }
\newcount\char_count  \char_count\z@

\def\print_char#1#2\end_line{%
   \print_first_char{#1}%
   \print_rest_of_line{#2}%
   }
{\obeytabs\gdef\@tab{^^I}}

\def\print_first_char#1{%
   \def\@char{#1}%
   \advance \char_count\@ne
   \ifx \@char\@tab  \print_tab
   \else  \@char
   \fi
   }
\newcount\count_mod_viii
\def\mod_viii#1{%
   \count@ #1\relax  \count_mod_viii\count@
   \divide \count@ 8\relax
   \multiply \count@ 8\relax
   \advance \count_mod_viii -\count@
   }
\def\print_tab{%
   \loop  \space \mod_viii\char_count
      \ifnum \count_mod_viii>\z@
	 \advance \char_count\@ne
   \repeat
   }
\def\print_rest_of_line#1{%
   \def\@line{#1}%
   \ifx \@line\empty  \char_count\z@
      \def\next##1\end_line{\relax}%
   \else  \let\next\print_char
   \fi
   \next#1\end_line
   }
{\catcode`\/=\escape		% / is temporary escape char
   \catcode`\\=\other
   /gdef/beginprog{%
      /gdef/end_verbatim_line{\endprog}%
      /begin_verbatim
      }
   /gdef/beginverbatim{%
      /gdef/end_verbatim_line{\endverbatim}%
      /begin_verbatim
      }
}				 % here \endgroup can't be used
\newskip\pre_sect_skip	\pre_sect_skip=2pc plus 1pc minus 6pt

\def\chap_intro{%
   \gdef\thesection{\arabic{section}.}%
   \gdef\chap_intro{\newpage}%
   \addvspace{\pre_sect_skip}%
   }

\def\sect_intro{%
   \gdef\thesection{\arabic{section}.}%
   \global\let\sect_intro\relax
   }
\def\chap#1.{%
   \chap_intro
   \@startsection{section}{1}%
      {\z@}%
      {\m@ne sp}%
      {\medskipamount}%
      {\normalsize\bf}%
      {#1.}%
   }
\def\sect{%
   \sect_intro
   \@startsection{section}{1}%
      {\z@}%
      {\pre_sect_skip}%
      {\m@ne sp}%
      {\normalsize\bf}%
      {\null}%
   }
\catcode`\_=\uscode

\endinput