Fixpoint forNats(n : nat) :=
  match n with
      | 0 => nil
      | S n' =>
        cons n' (forNats n')
  end.

Theorem fold_left_orb_true_in : forall (A : Type)(f : A -> bool)(ls : list A) a init, In a ls -> f a = true -> fold_left (fun b x => orb b (f x)) ls init = true.