Definition nminimal (work : nf_list) (ds : disjs) (ni : nested_imps)
  (ai : atomic_imps) (a : atoms) (context : flist) :=
  forall (c : form) (k : kripke_tree),
  Is_Monotone_kripke_tree k ->
  forces_ngamma work ds ni ai a k -> In c context -> forces_t k c.

Lemma nminimal_derivable_forces : forall (work : nf_list) (ds : disjs) (ni : nested_imps) (ai : atomic_imps) (a : atoms) (context : flist) (k : kripke_tree), Is_Monotone_kripke_tree k -> forces_ngamma work ds ni ai a k -> nminimal work ds ni ai a context -> forall c : form, Derivable context c -> forces_t k c.