intros work ds ni1 ni2 ai a context le sound.
unfold nsound in |- *.
intros c in_ngamma.
apply sound.
apply in_ngamma_le with ni2; assumption.
Qed.
(***********************************************************************)

(* Specifications: *)
Lemma nsound_shift_work_ds :
forall (i j : Int) (work : nf_list) (ds : disjs) (ni : nested_imps)
(ai : atomic_imps) (a : atoms) (context : flist),
nsound (NDisj i j :: work) ds ni ai a context ->
nsound work ((i, j) :: ds) ni ai a context.