Fixpoint BDDunivl (cfg : BDDconfig) (ul : list ad) 
 (node : ad) (ly : list BDDvar) {struct ly} : BDDconfig * ad :=
  match ly with
  | nil => (cfg, node)
  | y :: ly' =>
      match BDDunivl cfg ul node ly' with
      | (cfg1, node1) => BDDuniv cfg1 (node1 :: ul) y node1
      end
  end.

Lemma BDDunivl_used_nodes_preserved : used_nodes_preserved cfg (fst (BDDunivl cfg ul node ly)) ul.