public function addSubchainIndex($subchainIndex)
    {
        if (!$this->getSubchainIndexes()) {
            return $this->setSubchainIndexes(array($subchainIndex));
        } else {
            return $this->setSubchainIndexes(
                array_merge($this->getSubchainIndexes(), array($subchainIndex))
            );
        }
    }