apply nat_elim2 (\lambda n,m.n \leq m \to n-m = O).
intros.simplify.reflexivity.
intros.apply False_ind.
-(* ancora problemi con il not *)
-apply not_le_Sn_O n1 H.
+apply not_le_Sn_O.
+goal 13.apply H.
intros.
simplify.apply H.apply le_S_S_to_le. apply H1.
qed.