proposition: Theorem add_left_cancel (t a b : mynat) : t + a = t + b → a = b.
proof: Proof. intro f. rewrite add_comm in f. symmetry in f. rewrite add_comm in f. symmetry in f. apply add_right_cancel in f. exact f. Qed.
imports: Unicode.Utf8 Game.Mynat.Definition Game.Mynat.Add Game.World.Addition Game.World.Proposition Game.World.AdvancedProposition Setoid