proposition: Lemma subst_ee_open_ee : forall e1 e2 x u, expr u -> subst_ee x u (open_ee e1 e2) = open_ee (subst_ee x u e1) (subst_ee x u e2).
proof: Proof. Proof with auto. intros. unfold open_ee. apply subst_ee_open_ee_rec... Qed.
imports: Fsub.Fsub_LetSum_Definitions