\theoremstyle{plain}
\theoremheaderfont{\normalfont\bfseries\sc}
\theorembodyfont{\slshape}
-\theoremsymbol{\ensuremath{\diamondsuit}}
\theoremprework{\bigskip}
\theoremseparator{.}
\newtheorem{Def}{Définition}[chapter]
%\newtheorem{Pre}{Preuve}
+\theoremstyle{plain}
+\theoremheaderfont{\normalfont\sc}
+\theorembodyfont{\slshape}
+\theoremsymbol{$\dagger$}
+\theoremprework{\medskip}
+%\theorempostwork{}
+\theoremseparator{. }
+\newtheorem{Proof}{Preuve}%[chapter]
\theoremstyle{plain}
\theoremsymbol{$\dagger$}
\theoremprework{\medskip}
%\theorempostwork{}
-\theoremseparator{ :}
+\theoremseparator{. }
\newtheorem{Pre}{Preuve}%[chapter]
\theoremnumbering{greek}
\newtheorem{Lemma}{Lemme}[chapter]
-\theoremheaderfont{\sc}\theorembodyfont{\upshape}
-\theoremstyle{nonumberplain}
-\theoremseparator{}
-\theoremsymbol{\rule{1ex}{1ex}}
-\newtheorem{Proof}{Preuve}[chapter]
\theoremstyle{plain}