% Theorem environments.  The proof environment is the one amsthm defines; thmtools provides the restatable environment that macros/version.tex uses to restate a theorem in the appendix.
\usepackage{amsthm}

\theoremstyle{plain}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{corollary}[theorem]{Corollary}

\theoremstyle{definition}
\newtheorem{definition}[theorem]{Definition}

\usepackage{thmtools}