%theorem.sty
%Some changes to the definitions of
%the theorem environments and the addition of a proof.
% the additions....
\def\@begintheorem#1#2{\it \trivlist \item[\hskip \labelsep{\bf #1\ #2.}]}
\def\proof{{\bf Proof}. \ignorespaces}
\def\endproof{{\ \hfill\hbox{%
      \vrule width1.0ex height1.5ex
   }\parfillskip 0pt}\medskip}
\newtheorem{theorem}{Theorem}
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{corollary}[theorem]{Corollary}
\newtheorem{proposition}[theorem]{Proposition}
\newtheorem{definition}[theorem]{Definition}