intros x work ds ni1 ni2 ai a k forces.
unfold forces_ngamma in |- *.
intros c in_ngamma.
apply forces.
apply in_ngamma_shift_work_ni_x_ni; assumption.
Qed.
(********************************************************************)

Lemma forces_ngamma_cons_work_weak :
forall (b c : normal_form) (work : nf_list) (ds : disjs)
(ni : nested_imps) (ai : atomic_imps) (a : atoms)
(k : kripke_tree),
Is_Monotone_kripke_tree k ->
(forces_t k (nf2form b) -> forces_t k (nf2form c)) ->
forces_ngamma (b :: work) ds ni ai a k ->
forces_ngamma (c :: work) ds ni ai a k.