Return ``True`` if and only if the given ``index``
        is valid.

def _is_valid_index(self, index):
        """
        Return ``True`` if and only if the given ``index``
        is valid.
        """
        if isinstance(index, int):
            return (index >= 0) and (index < len(self))
        if isinstance(index, list):
            valid = True
            for i in index:
                valid = valid or self._is_valid_index(i)
            return valid
        return False