%% Undefine everithing
\let\theorem\relax
\let\endtheorem\relax
\let\corollary\relax
\let\endcorollary\relax
\let\lemma\relax
\let\endlemma\relax
\let\proposition\relax
\let\endproposition\relax
\let\assumption\relax
\let\endassumption\relax

\let\remark\relax
\let\endremark\relax
\let\example\relax
\let\endexample\relax
\let\defintion\relax
\let\enddefinition\relax
\let\example\relax
\let\endexample\relax

\newenvironment{theorem}{\begin{thm}}{\end{thm}}
\newenvironment{corollary}{\begin{cor}}{\end{cor}}
\newenvironment{lemma}{\begin{lem}}{\end{lem}}
\newenvironment{proposition}{\begin{prop}}{\end{prop}}
\newenvironment{assumption}{\begin{asm}}{\end{asm}}

\newenvironment{remark}{\begin{rem}}{\end{rem}}
\newenvironment{example}{\begin{exa}}{\end{exa}}
\newenvironment{definition}{\begin{defi}}{\end{defi}}