intros gamma work context a sound0.
unfold sound in |- *.
intros c in_gamma0.
apply sound0.
apply in_gamma_cons_gamma_tail; assumption.
Qed.

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