Definition nodes_preserved (cfg cfg' : BDDconfig) :=
  forall (x : BDDvar) (l r node : ad),
  MapGet _ (fst cfg) node = Some (x, (l, r)) ->
  MapGet _ (fst cfg') node = Some (x, (l, r)).

Lemma nodes_preserved_2 : forall (cfg cfg' : BDDconfig) (node : ad), config_node_OK cfg node -> nodes_preserved cfg cfg' -> config_node_OK cfg' node.