intros gamma work k a b forces_ab forces_gamma0.
apply forces_gamma_cons_gamma.
apply forces_ab.
apply forces_gamma_cons_gamma_head with gamma work; assumption.
apply forces_gamma_cons_gamma_tail with a; assumption.
Qed.

(* Specifications: *)
Lemma forces_gamma_cons_gamma_weak2 :
forall (gamma : flist) (work : nf_list) (k : kripke_tree) (a b c : form),
(forces_t k a -> forces_t k b -> forces_t k c) ->
forces_gamma (a :: b :: gamma) work k -> forces_gamma (c :: gamma) work k.