proposition: Lemma lazy_concat_in : forall {B : Type} (b : B) l ll, In_ll b l -> In_ll l ll -> In_ll b (concatLazyList ll).
proof: Proof. intros B b l ll Hb Hbll. induction ll; simpl in *; subst; auto. destruct Hbll as [Hal | Hinl]; subst; auto using lazy_append_in_l, lazy_append_in_r. Qed.
imports: ExtLib.Structures(Functor Applicative) List QuickChick(Tactics) Coq(ssreflect)