Remark forces_ngamma_del_ai :
 forall (i : Int) (work : nf_list) (ds : disjs) (ni : nested_imps)
   (ai ai' : atomic_imps) (a : atoms) (k : kripke_tree),
 EQUIV_DEL nf_list i ai ai' ->
 forces_ngamma work ds ni ai a k -> forces_ngamma work ds ni ai' a k.

Lemma forces_ngamma_eqv : forall (ni1 ni2 : nested_imps) (work : nf_list) (ds : disjs) (ai : atomic_imps) (a : atoms) (k : kripke_tree), eqv_ni ni1 ni2 -> forces_ngamma work ds ni2 ai a k -> forces_ngamma work ds ni1 ai a k.