Definition nsound (work : nf_list) (ds : disjs) (ni : nested_imps)
  (ai : atomic_imps) (a : atoms) (context : flist) :=
  forall c : normal_form,
  in_ngamma work ds ni ai a c -> Derivable context (nf2form c).

Lemma nsound_eqv : forall (work : nf_list) (ds : disjs) (ni1 ni2 : nested_imps) (ai : atomic_imps) (a : atoms) (context : flist), eqv_ni ni1 ni2 -> nsound work ds ni1 ai a context -> nsound work ds ni2 ai a context.