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

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.