intros ni1 ni2 work ds ai a k eqv12 forces2.
unfold forces_ngamma in |- *.
intros c in_ngamma.
apply forces2.
apply in_ngamma_eqv with ni1; assumption.
Qed.
(***********************************************************************)

Lemma forces_ngamma_shift_ds_work :
forall (i j : Int) (work : nf_list) (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 (NDisj i j :: work) ds ni ai a k.