code: def serialize(self, now=None):
          
          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
docs: 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