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_cons_gamma_cons_context : forall (gamma : flist) (work : nf_list) (context : flist) (a : form), minimal gamma work context -> minimal (a :: gamma) work (a :: context).