intros i j work ds ni ai a c in_ngamma0.
elim (in_ngamma_cons_work_rev (NDisj i j) work ds ni ai a c in_ngamma0);
clear in_ngamma0.
intros in_ngamma0.
apply in_ngamma_cons_ds_tail; assumption.
intros eq_c;  rewrite eq_c.
simpl in |- *.
apply in_ngamma_cons_ds_head.
Qed.

Lemma in_ngamma_shift_ds_work :
forall (i j : Int) (work : nf_list) (ds : disjs) (ni : nested_imps)
(ai : atomic_imps) (a : atoms) (c : normal_form),
in_ngamma work ((i, j) :: ds) ni ai a c ->
in_ngamma (NDisj i j :: work) ds ni ai a c.