Definition list_length_induction :
  forall P : list A -> Type,
  (forall l1 : list A,
   (forall l2 : list A, length l2 < length l1 -> P l2) -> P l1) ->
  forall l : list A, P l.

Theorem app_inv_head : forall l1 l2 l3 : list A, l1 ++ l2 = l1 ++ l3 -> l2 = l3.