-%To explain how the first phase works let us give an example. Consider
-%you want to prove the uniqueness of the identity element $0$ for natural
-%sum, and that you can rely on the previously demonstrated left
-%injectivity of the sum, that is $inj\_plus\_l:\forall x,y,z.x+y=z+y \to x =z$.
-%Typing
-%\begin{grafite}
-%theorem valid_name: \forall n,m. m + n = n \to m = O.
-% intros (n m H).
-%\end{grafite}