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_del_ai_rev : forall (i : Int) (bs work : nf_list) (ds : disjs) (ni : nested_imps) (ai ai' : atomic_imps) (a : atoms) (k : kripke_tree), LOOKUP nf_list i ai bs -> (forall (n : nat) (b : normal_form), my_nth normal_form n bs b -> forces_t k (nf2form (AImp i b))) -> EQUIV_DEL nf_list i ai ai' -> forces_ngamma work ds ni ai' a k -> forces_ngamma work ds ni ai a k.