proposition: Theorem mkRect_length: forall n m, length (mkRect n m) = ((n + 1) * (m + 1))%nat.
proof: Proof. intros n; elim n; simpl; auto. intros n1; rewrite <- app_nil_end; rewrite mkLine_length; rewrite Nat.add_0_r; auto. intros n1 Rec m1; rewrite length_app; rewrite Rec; rewrite mkLine_length; auto. Qed.
imports: ZArith Znumtheory Tactic Wf_nat ListAux UList FGroup EGroup IGroup