Remark nsound_del_ai :
 forall (i : Int) (work : nf_list) (ds : disjs) (ni : nested_imps)
   (ai ai' : atomic_imps) (a : atoms) (context : flist),
 EQUIV_DEL nf_list i ai ai' ->
 nsound work ds ni ai a context -> nsound work ds ni ai' a context.

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