intros l a b c i below_abc below_l.
elim below_abc; clear below_abc.
intros below_b below_c.
unfold below_list in |- *.
intros x in_x.
inversion_clear in_x.
rewrite <- H; assumption.
inversion_clear H.
rewrite <- H0; assumption.
apply below_l.
right; assumption.
apply below_l.
left; trivial.
Qed.
(********************************************************************)

Lemma subst_form_below :
forall (i : Int) (g a : form), below_form a i -> subst_form i g a = a.