Definition sound (Gamma : flist) (work : nf_list) (context : flist) :=
  forall a : form, in_gamma Gamma work a -> Derivable context a.

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