intros j a; case a; clear a.
apply le_n_Sn.
intros; apply le_trans with 2; apply le_n_Sn.
intros; apply le_n.
intros; apply le_n.
intros; apply le_n.
Qed.

Lemma weight_vimp :
forall (l : list Int) (a : form), weight (vimp l a) = weight a.