intros a gamma work k forces_gamma0.
unfold forces_gamma in |- *.
intros c in_gamma.
apply forces_gamma0.
apply in_gamma_shift_gamma_work; assumption.
Qed.
(*********************************************************************)

Lemma forces_gamma_cons_gamma_weak :
forall (gamma : flist) (work : nf_list) (k : kripke_tree) (a b : form),
(forces_t k a -> forces_t k b) ->
forces_gamma (a :: gamma) work k -> forces_gamma (b :: gamma) work k.