might not be minimal, but doesn't compile

\documentclass[12pt,final,notitlepage,onecolumn,german]{article}%
\usepackage{lscape}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{framed}
\usepackage{amsmath}
\usepackage{amsthm}%
\theoremstyle{definition}

\newtheorem{theo}{Theorem}
\newenvironment{theorem}[1][]
  {\begin{theo}[#1]\begin{leftbar}}
  {\end{leftbar}\end{theo}}

\newtheorem{lemma}[theorem]{Lemma}

\begin{document}
f00bar
\end{document}