proposition: Lemma fv_exp_subst_exp_notin : forall e1 e2 x1 x2, x2 `notin` fv_exp e1 -> x2 `notin` fv_exp e2 -> x2 `notin` fv_exp (subst_exp e2 x1 e1).
proof: Proof. pose proof fv_exp_subst_exp_notin_mutual as H; intuition eauto. Qed.
imports: Coq.Logic.FunctionalExtensionality Coq.Program.Equality Metalib.Metatheory Metalib.LibLNgen Stlc