proposition: Lemma subst_ren_term xi sigma t : subst sigma (ren xi t) = subst (funcomp sigma xi) t.
proof: Proof. elim: t xi sigma => /=. - done. - move=> ? IH1 ? IH2 ??. by rewrite IH1 IH2. - move=> ? IH ??. rewrite IH. congr lam. apply: ext_subst_term. by case. Qed.
imports: Undecidability.LambdaCalculus.Lambda PeanoNat Lia List Relations ssreflect