proposition: Lemma swap_term_is : forall M (pos: Pos M) (N: PlaceHolder pos), term (swap N) = swap_term pos (proj1_sig2 N).
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)