proposition: Lemma swap_env_eq : forall M (pos: Pos M) (N: PlaceHolder pos), env (swap N) = env M.
proof: Proof. intros M pos N. unfold swap. destruct (swap_aux N) as [Np [Nenv [Ntype Nterm]]]; try_solve. Qed.
imports: CoLoR(RelExtras ListExtras TermsBuilding)