In proof environment , “proof” doesn't show at the beginning of a proof in pdf

Question

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}

Answer 1

You need some text after \begin{solution}.

\documentclass{article}
\usepackage{amsthm}

\newtheoremstyle{solution}
  {}
  {}
  {\normalfont}
  {-10pt}
  {\bfseries}
  {:}
{0pt}%  {\newline}
  {}

\theoremstyle{definition}
\newtheorem*{lem}{Lemma}

\theoremstyle{solution}
\newtheorem*{solution}{Solution}

\begin{document}

\begin{description}
\item[1]
\begin{solution}\mbox{}

\begin{lem}
a lemma
\end{lem}

\begin{proof}
a proof
\end{proof}

\end{solution}
\end{description}

\end{document}