intros a l i below_a below_l.
unfold below_list in |- *.
intros b in_b.
inversion_clear in_b.
rewrite <- H; assumption.
apply below_l; assumption.
Qed.

Lemma below_list_weak :
forall (l : flist) (a b : form) (i : Int),
(below_form a i -> below_form b i) ->
below_list (a :: l) i -> below_list (b :: l) i.