proposition: Lemma mask_app_2 f βs l1 l2 : mask f βs (l1 ++ l2) = mask f (take (length l1) βs) l1 ++ mask f (drop (length l1) βs) l2.
proof: Proof. revert βs. induction l1; intros [|??]; f_equal/=; auto. Qed.
imports: stdpp(options)