intros l a b i below_ab below_l.
unfold below_list in |- *.
intros x in_x.
inversion_clear in_x.
rewrite <- H; clear H x.
apply below_ab.
apply below_l.
left; trivial.
apply below_l.
right; assumption.
Qed.

Lemma below_list_weak2 :
forall (l : flist) (a b c : form) (i : Int),
(below_form a i -> below_form b i /\ below_form c i) ->
below_list (a :: l) i -> below_list (b :: c :: l) i.