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Remark nsound_app_work :
 forall (bs work : nf_list) (ds : disjs) (ni : nested_imps)
   (ai : atomic_imps) (a : atoms) (context : flist),
 (forall (n : nat) (b : normal_form),
  my_nth normal_form n bs b -> Derivable context (nf2form b)) ->
 nsound work ds ni ai a context -> nsound (bs ++ work) ds ni ai a context.

Lemma nsound_cons_ds_tail : forall (work : nf_list) (i j : Int) (ds : disjs) (ni : nested_imps) (ai : atomic_imps) (a : atoms) (context : flist), nsound work ((i, j) :: ds) ni ai a context -> nsound work ds ni ai a context.