Remark forces_ngamma_app_work :
 forall (bs work : nf_list) (ds : disjs) (ni : nested_imps)
   (ai : atomic_imps) (a : atoms) (k : kripke_tree),
 (forall (n : nat) (b : normal_form),
  my_nth normal_form n bs b -> forces_t k (nf2form b)) ->
 forces_ngamma work ds ni ai a k -> forces_ngamma (bs ++ work) ds ni ai a k.

Lemma forces_ngamma_cons_ni_tail : forall (work : nf_list) (ds : disjs) (x : nested_imp) (ni : nested_imps) (ai : atomic_imps) (a : atoms) (k : kripke_tree), forces_ngamma work ds (x :: ni) ai a k -> forces_ngamma work ds ni ai a k.