intros gamma work context a sound0.
apply sound0.
apply in_gamma_cons_gamma_head; assumption.
Qed.
(**********************************************************************)

Lemma sound_shift_gamma_work :
forall (a : normal_form) (gamma : flist) (work : nf_list) (context : flist),
sound (nf2form a :: gamma) work context -> sound gamma (a :: work) context.