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

Lemma length_mul_split A k (K:(0<k)%nat) n (N:(0<n)%nat): forall (l:list A), length l = (k * n)%nat -> exists l1, exists l2, l=l1++l2 /\ length l1=n /\ length l2 = ((k-1) * n)%nat.