intros gamma work k a forces_gamma0.
apply forces_gamma0.
apply in_gamma_cons_gamma_head; assumption.
Qed.
(*********************************************************************)

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