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

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