Warning: this is an htmlized version!
The original is across this link,
and the conversion rules are here.
%	proof.sty	(Proof Figure Macros)
%
% 	version 3.1 (for both LaTeX 2.09 and LaTeX 2e)
%	Nov 24, 2005
% 	Copyright (C) 1990 -- 2005, Makoto Tatsuta (tatsuta@nii.ac.jp)
% 
% This program is free software; you can redistribute it or modify
% it under the terms of the GNU General Public License as published by
% the Free Software Foundation; either versions 1, or (at your option)
% any later version.
% 
% This program is distributed in the hope that it will be useful
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU General Public License for more details.
%
%	Usage:
%		In \documentstyle, specify an optional style `proof', say,
%			\documentstyle[proof]{article}.
%
%	The following macros are available:
%
%	In all the following macros, all the arguments such as
%	<Lowers> and <Uppers> are processed in math mode.
%
%	\infer<Lower><Uppers>
%		draws an inference.
%
%		Use & in <Uppers> to delimit upper formulae.
%		<Uppers> consists more than 0 formulae.
%
%		\infer returns \hbox{ ... } or \vbox{ ... } and
%		sets \@LeftOffset and \@RightOffset globally.
%
%	\infer[<Label>]<Lower><Uppers>
%		draws an inference labeled with <Label>.
%
%	\infer*<Lower><Uppers>
%		draws a many step deduction.
%
%	\infer*[<Label>]<Lower><Uppers>
%		draws a many step deduction labeled with <Label>.
%
%	\infer=<Lower><Uppers>
%		draws a double-ruled deduction.
%
%	\infer=[<Label>]<Lower><Uppers>
%		draws a double-ruled deduction labeled with <Label>.
%
%	\deduce<Lower><Uppers>
%		draws an inference without a rule.
%
%	\deduce[<Proof>]<Lower><Uppers>
%		draws a many step deduction with a proof name.
%
%	Example:
%		If you want to write
%       	       	    B C
%		 	   -----
%		       A     D
%		      ----------
%			  E
%	use
%		\infer{E}{
%			A
%			&
%			\infer{D}{B & C}
%		}
%

%	Style Parameters

\newdimen\inferLineSkip		\inferLineSkip=2pt
\newdimen\inferLabelSkip	\inferLabelSkip=5pt
\def\inferTabSkip{\quad}

%	Variables

\newdimen\@LeftOffset	% global
\newdimen\@RightOffset	% global
\newdimen\@SavedLeftOffset	% safe from users

\newdimen\UpperWidth
\newdimen\LowerWidth
\newdimen\LowerHeight
\newdimen\UpperLeftOffset
\newdimen\UpperRightOffset
\newdimen\UpperCenter
\newdimen\LowerCenter
\newdimen\UpperAdjust
\newdimen\RuleAdjust
\newdimen\LowerAdjust
\newdimen\RuleWidth
\newdimen\HLabelAdjust
\newdimen\VLabelAdjust
\newdimen\WidthAdjust

\newbox\@UpperPart
\newbox\@LowerPart
\newbox\@LabelPart
\newbox\ResultBox

%	Flags

\newif\if@inferRule	% whether \@infer draws a rule.
\newif\if@DoubleRule	% whether \@infer draws doulbe rules.
\newif\if@ReturnLeftOffset	% whether \@infer returns \@LeftOffset.

%	Special Fonts

\def\DeduceSym{\vtop{\baselineskip4\p@ \lineskiplimit\z@
    \vbox{\hbox{.}\hbox{.}\hbox{.}}\hbox{.}}}

%	Macros

% Renaming @ifnextchar and @ifnch of LaTeX2e to @IFnextchar and @IFnch.

\def\@IFnextchar#1#2#3{%
  \let\reserved@e=#1\def\reserved@a{#2}\def\reserved@b{#3}\futurelet
    \reserved@c\@IFnch}
\def\@IFnch{\ifx \reserved@c \@sptoken \let\reserved@d\@xifnch
      \else \ifx \reserved@c \reserved@e\let\reserved@d\reserved@a\else
          \let\reserved@d\reserved@b\fi
      \fi \reserved@d}

\def\@ifEmpty#1#2#3{\def\@tempa{\@empty}\def\@tempb{#1}\relax
	\ifx \@tempa \@tempb #2\else #3\fi }

\def\infer{\@IFnextchar *{\@inferSteps}{\relax
	\@IFnextchar ={\@inferDoubleRule}{\@inferOneStep}}}

\def\@inferOneStep{\@inferRuletrue \@DoubleRulefalse
	\@IFnextchar [{\@infer}{\@infer[\@empty]}}

\def\@inferDoubleRule={\@inferRuletrue \@DoubleRuletrue
	\@IFnextchar [{\@infer}{\@infer[\@empty]}}

\def\@inferSteps*{\@IFnextchar [{\@@inferSteps}{\@@inferSteps[\@empty]}}

\def\@@inferSteps[#1]{\@deduce{#1}[\DeduceSym]}

\def\deduce{\@IFnextchar [{\@deduce{\@empty}}
	{\@inferRulefalse \@infer[\@empty]}}

%	\@deduce<Proof Label>[<Proof>]<Lower><Uppers>

\def\@deduce#1[#2]#3#4{\@inferRulefalse
	\@infer[\@empty]{#3}{\@infer[{#1}]{#2}{#4}}}

%	\@infer[<Label>]<Lower><Uppers>
%		If \@inferRuletrue, it draws a rule and <Label> is right to
%		a rule. In this case, if \@DoubleRuletrue, it draws
%		double rules.
%
%		Otherwise, draws no rule and <Label> is right to <Lower>.

\def\@infer[#1]#2#3{\relax
% Get parameters
	\if@ReturnLeftOffset \else \@SavedLeftOffset=\@LeftOffset \fi
	\setbox\@LabelPart=\hbox{$#1$}\relax
	\setbox\@LowerPart=\hbox{$#2$}\relax
%
	\global\@LeftOffset=0pt
	\setbox\@UpperPart=\vbox{\tabskip=0pt \halign{\relax
		\global\@RightOffset=0pt \@ReturnLeftOffsettrue $##$&&
		\inferTabSkip
		\global\@RightOffset=0pt \@ReturnLeftOffsetfalse $##$\cr
		#3\cr}}\relax
	\UpperLeftOffset=\@LeftOffset
	\UpperRightOffset=\@RightOffset
% Calculate Adjustments
	\LowerWidth=\wd\@LowerPart
	\LowerHeight=\ht\@LowerPart
	\LowerCenter=0.5\LowerWidth
%
	\UpperWidth=\wd\@UpperPart \advance\UpperWidth by -\UpperLeftOffset
	\advance\UpperWidth by -\UpperRightOffset
	\UpperCenter=\UpperLeftOffset
	\advance\UpperCenter by 0.5\UpperWidth
%
	\ifdim \UpperWidth > \LowerWidth
		% \UpperCenter > \LowerCenter
	\UpperAdjust=0pt
	\RuleAdjust=\UpperLeftOffset
	\LowerAdjust=\UpperCenter \advance\LowerAdjust by -\LowerCenter
	\RuleWidth=\UpperWidth
	\global\@LeftOffset=\LowerAdjust
%
	\else	% \UpperWidth <= \LowerWidth
	\ifdim \UpperCenter > \LowerCenter
%
	\UpperAdjust=0pt
	\RuleAdjust=\UpperCenter \advance\RuleAdjust by -\LowerCenter
	\LowerAdjust=\RuleAdjust
	\RuleWidth=\LowerWidth
	\global\@LeftOffset=\LowerAdjust
%
	\else	% \UpperWidth <= \LowerWidth
		% \UpperCenter <= \LowerCenter
%
	\UpperAdjust=\LowerCenter \advance\UpperAdjust by -\UpperCenter
	\RuleAdjust=0pt
	\LowerAdjust=0pt
	\RuleWidth=\LowerWidth
	\global\@LeftOffset=0pt
%
	\fi\fi
% Make a box
	\if@inferRule
%
	\setbox\ResultBox=\vbox{
		\moveright \UpperAdjust \box\@UpperPart
		\nointerlineskip \kern\inferLineSkip
		\if@DoubleRule
		\moveright \RuleAdjust \vbox{\hrule width\RuleWidth
			\kern 1pt\hrule width\RuleWidth}\relax
		\else
		\moveright \RuleAdjust \vbox{\hrule width\RuleWidth}\relax
		\fi
		\nointerlineskip \kern\inferLineSkip
		\moveright \LowerAdjust \box\@LowerPart }\relax
%
	\@ifEmpty{#1}{}{\relax
%
	\HLabelAdjust=\wd\ResultBox	\advance\HLabelAdjust by -\RuleAdjust
	\advance\HLabelAdjust by -\RuleWidth
	\WidthAdjust=\HLabelAdjust
	\advance\WidthAdjust by -\inferLabelSkip
	\advance\WidthAdjust by -\wd\@LabelPart
	\ifdim \WidthAdjust < 0pt \WidthAdjust=0pt \fi
%
	\VLabelAdjust=\dp\@LabelPart
	\advance\VLabelAdjust by -\ht\@LabelPart
	\VLabelAdjust=0.5\VLabelAdjust	\advance\VLabelAdjust by \LowerHeight
	\advance\VLabelAdjust by \inferLineSkip
%
	\setbox\ResultBox=\hbox{\box\ResultBox
		\kern -\HLabelAdjust \kern\inferLabelSkip
		\raise\VLabelAdjust \box\@LabelPart \kern\WidthAdjust}\relax
%
	}\relax % end @ifEmpty
%
	\else % \@inferRulefalse
%
	\setbox\ResultBox=\vbox{
		\moveright \UpperAdjust \box\@UpperPart
		\nointerlineskip \kern\inferLineSkip
		\moveright \LowerAdjust \hbox{\unhbox\@LowerPart
			\@ifEmpty{#1}{}{\relax
			\kern\inferLabelSkip \unhbox\@LabelPart}}}\relax
	\fi
%
	\global\@RightOffset=\wd\ResultBox
	\global\advance\@RightOffset by -\@LeftOffset
	\global\advance\@RightOffset by -\LowerWidth
	\if@ReturnLeftOffset \else \global\@LeftOffset=\@SavedLeftOffset \fi
%
	\box\ResultBox
}