Corollary Forall_sublist_first : forall {A} {d : Inhabitant A} (P : A -> Prop) i j l
  (Hrangei : 0 <= i <= Zlength l) (Hi : Forall P (sublist 0 i l)) (Hi' : ~P (Znth i l))
  (Hrangej : 0 <= j <= Zlength l) (Hj : Forall P (sublist 0 j l)) (Hj' : ~P (Znth j l)),
  i = j.

Lemma app_eq_inv : forall {A} (l1 l2 l3 l4 : list A) (Heq : l1 ++ l2 = l3 ++ l4) (Hlen : length l1 = length l3), l1 = l3 /\ l2 = l4.