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_shift_work_ni_x_ni : forall (x : nested_imp) (work : nf_list) (ds : disjs) (ni1 ni2 : nested_imps) (ai : atomic_imps) (a : atoms) (k : kripke_tree), forces_ngamma (NImp_NF (nested_imp2nimp x) :: work) ds (ni1 ++ ni2) ai a k -> forces_ngamma work ds (ni1 ++ x :: ni2) ai a k.