intros a gamma work context sound0.
unfold sound in |- *.
intros c in_gamma.
apply sound0.
apply in_gamma_shift_gamma_work; assumption.
Qed.
(**********************************************************************)

Lemma sound_cons_gamma_cons_context :
forall (gamma : flist) (work : nf_list) (context : flist) (a : form),
sound gamma work context -> sound (a :: gamma) work (a :: context).