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_shift_work_ni_x_ni : forall (x : nested_imp) (work : nf_list) (ds : disjs) (ni1 ni2 : nested_imps) (ai : atomic_imps) (a : atoms) (context : flist), nsound (NImp_NF (nested_imp2nimp x) :: work) ds (ni1 ++ ni2) ai a context -> nsound work ds (ni1 ++ x :: ni2) ai a context.