intros bs work ds ni ai a k forces.
unfold forces_ngamma in |- *.
intros c in_ngamma.
apply forces.
apply in_ngamma_work_app1; assumption.
Qed.

Lemma forces_ngamma_cons_ds_tail :
forall (work : nf_list) (i j : Int) (ds : disjs) (ni : nested_imps)
(ai : atomic_imps) (a : atoms) (k : kripke_tree),
forces_ngamma work ((i, j) :: ds) ni ai a k ->
forces_ngamma work ds ni ai a k.