intros work i j ds ni ai a.
apply In_Disjs with 0.
apply My_NthO.
Qed.

Lemma in_ngamma_cons_ds_rev :
forall (work : nf_list) (i j : Int) (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 work ds ni ai a c + {c = NDisj i j}.