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

Lemma forces_gamma_shift_work_gamma : forall (a : normal_form) (gamma : flist) (work : nf_list) (k : kripke_tree), forces_gamma gamma (a :: work) k -> forces_gamma (nf2form a :: gamma) work k.