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 combine_app : forall {A B} (l1 l2 : list A) (l1' l2' : list B), length l1 = length l1' -> combine (l1 ++ l2) (l1' ++ l2') = combine l1 l1' ++ combine l2 l2'.