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

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.