proposition: Lemma subst_exp_fresh_eq_mutual : (forall e2 e1 x1, x1 `notin` fv_exp e2 -> subst_exp e1 x1 e2 = e2).
proof: Proof. apply_mutual_ind exp_mutind; default_simp. Qed.
imports: Coq.Logic.FunctionalExtensionality Coq.Program.Equality Metalib.Metatheory Metalib.LibLNgen Stlc