<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def serialize(self, now=None):
 """ Serialize this SubSection and all children to lxml Element and return it. :param str now: Default value for CREATED if none set :return: dmdSec/techMD/rightsMD/sourceMD/digiprovMD Element with all children """

        created = self.created if self.created is not None else now
        el = etree.Element(utils.lxmlns("mets") + self.subsection, ID=self.id_string)
        if created:  # Don't add CREATED if none was parsed
            el.set("CREATED", created)
        status = self.get_status()
        if status:
            el.set("STATUS", status)
        if self.contents:
            el.append(self.contents.serialize())
        return el