proposition: Lemma belongs_ok_concat_inv_r : forall x F G, ok (F & G) -> x ∈ G -> x ∉ F.
proof: Proof. intros x F G Hok HinG. apply ok_concat_inv in Hok. destruct Hok as [ HokE [ HokF [ HdomE HdomF ] ] ]. unfold all_notin, notin, belongs in *; auto. Qed.
imports: Utf8 Equalities