intros work i j ds ni ai a k forces.
unfold forces_ngamma in |- *.
intros c in_ngamma.
apply forces.
apply in_ngamma_cons_ds_tail; assumption.
Qed.

Lemma forces_ngamma_cons_ni_tail :
forall (work : nf_list) (ds : disjs) (x : nested_imp)
(ni : nested_imps) (ai : atomic_imps) (a : atoms)
(k : kripke_tree),
forces_ngamma work ds (x :: ni) ai a k -> forces_ngamma work ds ni ai a k.