def METHOD_NAME(self, idx_spec):
    """
    Parameters
    ----------
    idx_spec : str or None
        A string in the form of a pair of brackets enclosing some
        index expression matching data objects in the file
    Returns
    -------
    result : iterable
        An iterable of indexes that can be used to access each data object
        matching the pattern. Should contain one index if the
        `idx_spec` was empty or None (some default) or an empty sequence
        if no indexes could be found matching the pattern.
    """
    raise NotImplementedError("subclass should override this")