Definition Forall_tl (A : Type) (P : A -> Prop) (a : A) (l : list A)
           (H : Forall P (a :: l)): Forall P l.

Lemma skipn_app2: forall A n (al bl: list A), (n >= length al)%nat -> skipn n (al++bl) = skipn (n-length al) bl.