intros l.
elim l; clear l.
intro a; trivial.
intros i l ih a.
simpl in |- *.
apply ih with (a := Imp (Atom i) a).
Qed.

Lemma weight_gamma_weak :
forall (a b : form) (gamma : flist) (n : nat),
weight a < weight b ->
weight_gamma (b :: gamma) < S n -> weight_gamma (a :: gamma) < n.