+(**************************************************************************)
+(* ___ *)
+(* ||M|| *)
+(* ||A|| A project by Andrea Asperti *)
+(* ||T|| *)
+(* ||I|| Developers: *)
+(* ||T|| The HELM team. *)
+(* ||A|| http://helm.cs.unibo.it *)
+(* \ / *)
+(* \ / This file is distributed under the terms of the *)
+(* v GNU General Public License Version 2 *)
+(* *)
+(**************************************************************************)
+
+
+
+include "nat/minus.ma".
+
+theorem p2: \forall x,y:nat. x+x = (S(S O))*x.
+intros.
+demodulate by times_n_Sm times_n_O sym_times plus_n_O.
+qed.
+
+theorem p4: \forall x:nat. (x+(S O))*(x-(S O))=x*x - (S O).
+intro.
+apply (nat_case x)
+[simplify.reflexivity
+|intro.demodulate.reflexivity]
+qed.
+
+theorem p5: \forall x,y:nat. (x+y)*(x+y) = x*x + (S(S O))*(x*y) + (y*y).
+intros.demodulate.reflexivity.
+qed.
+