docs: def get_xml(self, pdb_id):
          ''' Returns the XML for pdb_id if the tag exists.'''
          self.tmp_string = None
          context = etree.iterparse(io.BytesIO(self.xml_contents), events=('end',), tag=self.PDBTM_entry_tag_type)
          try:
              fast_iter(context, self._get_xml, pdb_id = pdb_id.upper())
          except EarlyOut: pass
          return self.tmp_string