proposition: Lemma tFresh_app_l : forall g l1 l2 ts, @tFresh G vc g (l1 ++ l2) ts <=> (tFresh l1 ts # tFresh l2 ts # disjoint l1 l2 # no_repeats l1 # no_repeats l2).
proof: Proof. unfold tFresh; introv. rw disjoint_app_l. rw no_repeats_app; split; sp. Qed.
imports: AlphaEquality SwapProps Eqdep_dec