def n_missing(s):
    """Get the number of missing feature subsets.

    Parameters
    ----------
    s : pandas.Series
        Records of feature subsets.

    Returns
    -------
    int

    See also
    --------
    subrela.records.iterate_missing : Iterate missing feature subsets.

    Examples
    --------
    >>> s = from_arrays([[True, False, False], [True, False, True]],
    ...                 [0.2, 0.8],
    ...                 features=['A', 'B', 'C'])
    >>> s
    A     B      C    
    True  False  False    0.2
                 True     0.8
    dtype: float64
    >>> n_missing(s)
    5
    """
    return 2**s.index.nlevels - 1 - len(s)