induction l; simpl; intros. constructor.
econstructor. 2: reflexivity.
apply Forall_inv in F; trivial.
apply IHl. apply Forall_tl in F; trivial.
Qed.

(* Specifications: *)
Lemma concat_app {A} (l1 l2:list A): l1 ++ l2 = l1 ++ concat (l2 :: nil).
Definition splitList {A : Type} (n : nat) (l : list A) : (list A * list A) :=
(firstn n l, skipn n l).