def _stringify(self, transform: StringifyTransform) -> str:
    res = ['(']
    res.append(transform(self.inner))
    res.append(')')
    res.append(transform(self.next))
    return ''.join(res)
def _stringify(self, transform: StringifyTransform) -> str:
    res = ['(']
    res.append(transform(self.inner))
    res.append(')')
    res.append(transform(self.next))
    return .join(res)