eclbkbox.sty

% eclbkbox.sty by Hideki Isozaki, 1992
% Date: May  28, 1993

\newbox\bk@bxb
\newbox\bk@bxa
\newif\if@bkcont 
\newif\ifbkcount
\newcount\bk@lcnt

\def\breakboxskip{2pt}
\def\breakboxparindent{1.8em}

\def\breakbox{\vskip\breakboxskip\relax
\setbox\bk@bxb\vbox\bgroup
\advance\linewidth -2\fboxrule
\advance\linewidth -2\fboxsep
\hsize\linewidth\@parboxrestore
\parindent\breakboxparindent\relax}

% \@tempdimb: amount of vertical skip 
% between the first line (\bk@bxa) and the rest (\bk@bxb)
\def\bk@split{%
\@tempdimb\ht\bk@bxb % height of original box
\advance\@tempdimb\dp\bk@bxb 
\setbox\bk@bxa\vsplit\bk@bxb to\z@ % split it
\setbox\bk@bxa\vbox{\unvbox\bk@bxa}% recover height & depth of \bk@bxa
\setbox\@tempboxa\vbox{\copy\bk@bxa\copy\bk@bxb}% naive concatenation
\advance\@tempdimb-\ht\@tempboxa 
\advance\@tempdimb-\dp\@tempboxa}% gap between two boxes
 

% \@tempdima: height of the first line (\bk@bxa) + fboxsep
\def\bk@addfsepht{%
     \setbox\bk@bxa\vbox{\vskip\fboxsep\box\bk@bxa}}

\def\bk@addskipht{%
     \setbox\bk@bxa\vbox{\vskip\@tempdimb\box\bk@bxa}}

% \@tempdima: depth of the first line (\bk@bxa) + fboxsep
\def\bk@addfsepdp{%
     \@tempdima\dp\bk@bxa
     \advance\@tempdima\fboxsep
     \dp\bk@bxa\@tempdima}

% \@tempdima: depth of the first line (\bk@bxa) + vertical skip
\def\bk@addskipdp{%
     \@tempdima\dp\bk@bxa
     \advance\@tempdima\@tempdimb
     \dp\bk@bxa\@tempdima}

\def\bk@line{%
    \hbox to \linewidth{\ifbkcount\smash{\llap{\the\bk@lcnt\ }}\fi
    \vrule \@width\fboxrule\hskip\fboxsep
    \box\bk@bxa\hfil
    \hskip\fboxsep\vrule \@width\fboxrule}}

\def\endbreakbox{\egroup
\ifhmode\par\fi{\noindent\bk@lcnt\@ne 
\@bkconttrue\baselineskip\z@\lineskiplimit\z@
\lineskip\z@\vfuzz\maxdimen
\bk@split\bk@addfsepht\bk@addskipdp
\ifvoid\bk@bxb      % Only one line
\def\bk@fstln{\bk@addfsepdp
\vbox{\hrule\@height\fboxrule\bk@line\hrule\@height\fboxrule}}%
\else               % More than one line
\def\bk@fstln{\vbox{\hrule\@height\fboxrule\bk@line}\hfil
\advance\bk@lcnt\@ne
\loop 
 \bk@split\bk@addskipdp\leavevmode
\ifvoid\bk@bxb      % The last line
 \@bkcontfalse\bk@addfsepdp
 \vtop{\bk@line\hrule\@height\fboxrule}%
\else               % 2,...,(n-1)
 \bk@line
\fi
 \hfil\advance\bk@lcnt\@ne
\if@bkcont\repeat}%
\fi
\leavevmode\bk@fstln\par}\vskip\breakboxskip\relax}

\bkcountfalse