proposition: Lemma fv_exp_subst_exp_lower_mutual : (forall e1 e2 x1, remove x1 (fv_exp e1) [<=] fv_exp (subst_exp e2 x1 e1)).
proof: Proof. apply_mutual_ind exp_mutind; default_simp; fsetdec. Qed.
imports: Coq.Logic.FunctionalExtensionality Coq.Program.Equality Metalib.Metatheory Metalib.LibLNgen Stlc