intros.
apply (weight_gamma_weak2 a b c gamma n); assumption.
Qed.

Lemma weight_gamma_weak3 :
forall (a b c d : form) (gamma : flist) (n : nat),
weight a + weight b + weight c < weight d ->
weight_gamma (d :: gamma) < S n -> weight_gamma (a :: b :: c :: gamma) < n.