def _get_pdb_id(self, elem, **kwargs):
        '''If self.restrict_to_transmembrane_proteins is False then this adds all ids to self.ids. Otherwise, only transmembrane protein ids are added.'''
        id = elem.attrib['ID']
        if self.restrict_to_transmembrane_proteins:
            tmp = elem.attrib['TMP']
            assert(tmp == 'no' or tmp == 'yes' or tmp == 'not')
            if tmp == 'yes':
                self.ids[id] = PDBTM._get_tm_type(elem)
        else:
            self.ids[id] = self.ids.get(id, 0) + 1