private function checkReleasetype()
    {
        // check Releasetype
        if (null === $this->releasetype) {
            throw new BuildException('releasetype attribute is required', $this->getLocation());
        }
        // known releasetypes
        $releaseTypes = [
            self::RELEASETYPE_MAJOR,
            self::RELEASETYPE_MINOR,
            self::RELEASETYPE_BUGFIX
        ];

        if (!in_array($this->releasetype, $releaseTypes)) {
            throw new BuildException(
                sprintf(
                    'Unknown Releasetype %s..Must be one of Major, Minor or Bugfix',
                    $this->releasetype
                ),
                $this->getLocation()
            );
        }
    }