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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def serialize(self, now=None):
 """ Serialize this amdSec and all children to lxml Element and return it. :param str now: Default value for CREATED in children if none set :return: amdSec Element with all children """

        if self._tree is not None:
            return self._tree
        el = etree.Element(utils.lxmlns("mets") + self.tag, ID=self.id_string)
        self.subsections.sort()
        for child in self.subsections:
            el.append(child.serialize(now))
        return el