intros c work ds ni ai a k forces_a forces_ngamma0.
unfold forces_ngamma in |- *.
intros c0 in_ngamma.
elim (in_ngamma_cons_work_rev c work ds ni ai a c0 in_ngamma);
clear in_ngamma.
intros in_ngamma.
apply forces_ngamma0; assumption.
intros eq_c0_c;  rewrite eq_c0_c; assumption.
Qed.

Lemma forces_ngamma_cons_work_tail :
forall (c : normal_form) (work : nf_list) (ds : disjs)
(ni : nested_imps) (ai : atomic_imps) (a : atoms)
(k : kripke_tree),
forces_ngamma (c :: work) ds ni ai a k -> forces_ngamma work ds ni ai a k.