def to_spans(self):
        s = set()
        self._convert_to_spans(self.tree, 1, s)
        return s
No child elements, child elements not allowed.
dataset/ETHPy150Open emilmont/pyStatParser/stat_parser/eval_parser.py/TreeOperations.to_spans--crossover with--dataset/ETHPy150Open typesupply/woffTools/Lib/woffTools/test/test_validate.py/metadataAbstractElementIllegalChildElementTest1