proposition: Theorem mul_pos (a b : mynat) : a ≠ 0 → b ≠ 0 → a * b ≠ 0.
proof: Proof. intro f. intro g. intro h. destruct b. apply g. reflexivity. apply f. rewrite mul_succ in h. exact (add_left_eq_zero h). Qed.
imports: Unicode.Utf8 Game.Mynat.Definition Game.Mynat.Add Game.Mynat.Mul Game.World.Addition Game.World.Multiplication Game.World.Proposition Game.World.AdvancedProposition Game.World.AdvancedAddition