proposition: Lemma elements_node : forall (t1 t2:t elt) x e z l, elements t1 ++ (x,e) :: elements t2 ++ l = elements (Node t1 x e t2 z) ++ l.
proof: Proof. unfold elements; simpl; intros. rewrite !elements_app, !app_nil_r, <- !app_assoc; auto. Qed.
imports: FunInd FMapInterface FMapList ZArith Int