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_a : forall (i : Int) (work : nf_list) (ds : disjs) (ni : nested_imps) (ai : atomic_imps) (a a' : atoms) (context : flist), EQUIV_INS unit i (fun _ : unit => tt) tt a a' -> nsound (NAtom i :: work) ds ni ai a context -> nsound work ds ni ai a' context.