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_shift_gamma_work : forall (a : normal_form) (gamma : flist) (work : nf_list) (context : flist), minimal (nf2form a :: gamma) work context -> minimal gamma (a :: work) context.