Definition minimal (gamma : flist) (work : nf_list) 
  (context : flist) :=
  forall (a : form) (k : kripke_tree),
  Is_Monotone_kripke_tree k ->
  forces_gamma gamma work k -> In a context -> forces_t k a.

Lemma minimal_derivable_forces : forall (gamma : flist) (work : nf_list) (context : flist) (k : kripke_tree), Is_Monotone_kripke_tree k -> forces_gamma gamma work k -> minimal gamma work context -> forall a : form, Derivable context a -> forces_t k a.