intros gamma work k a forces_a forces_gamma0.
unfold forces_gamma in |- *.
intros c in_gamma0.
elim (in_gamma_cons_gamma_rev a gamma work c in_gamma0); clear in_gamma0.
intros in_gamma0.
apply forces_gamma0; assumption.
intros eq_c;  rewrite eq_c; assumption.
Qed.

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