protected function registerCopy(PersistentObject $origNode, PersistentObject $copyNode) {
    // store oid in the registry
    $registry = $this->getLocalSessionValue(self::OBJECT_MAP_VAR);
    $registry[$origNode->getOID()->__toString()] = $copyNode->getOID()->__toString();
    $this->setLocalSessionValue(self::OBJECT_MAP_VAR, $registry);
  }