intros.
simpl in |- *.
apply lt_plus_assoc_l.
apply lt_S_n.
apply le_lt_trans with (weight_gamma (c :: gamma)); try assumption.
simpl in |- *.
apply lt_le_S.
apply plus_lt_compat_r.
assumption.
Qed.

Lemma weight_gamma_weak2' :
forall (a b c : form) (gamma : flist) (n : nat),
weight a + weight b < weight c ->
weight c + weight_gamma gamma < S n ->
weight a + (weight b + weight_gamma gamma) < n.