protected function _init($definition=null)
    {

        if (!is_null($definition)) {
            $this->_packageXml = simplexml_load_string($definition);
        } else {
            $packageXmlStub = <<<END
<?xml version="1.0"?>
<package>
    <name />
    <version />
    <stability />
    <license />
    <channel />
    <extends />
    <summary />
    <description />
    <notes />
    <authors />
    <date />
    <time />
    <contents />
    <compatible />
    <dependencies />
</package>
END;
            $this->_packageXml = simplexml_load_string($packageXmlStub);
        }
        return $this;
    }