I use a proof environment in a solution environment which inside a description environment. But after compiling my tex file, in the pdf file the proof doesn't show at the beginning of a proof.
"solution" is a user-defined theoremstyle whose parameters are as follow:
\newtheoremstyle{solution}%
{}{}%
{}{-10pt}%
{\bfseries}{:}%
{\newline}%
{}
Except this, I don't make any changes.
Something relevant in my preamble:
\theoremstyle{definition}
\newtheorem*{lem}{Lemma}
What I did in my tex file is as follow:
\begin{description}
\item[1]
\begin{solution}
\begin{lem}
a lemma
\end{lem}
\begin{proof}
a proof
\end{proof}
\end{solution}
\end{description}