def multif0_to_timefreq(times, freqs):
    """Unroll a multif0 annotation of the form (t, [f1, f2, f3])
    to a list of (t, f) where t may be repeated.

    Parameters
    ----------
    times : list
        Time stamps
    freqs : list of lists
        all frequencies for a given time stamp

    Returns
    -------
    t_unrolled : list
        Unrolled time stamps
    f_unrolled : list
        Unrolled frequency values

    """
    t_unrolled = []
    f_unrolled = []
    for t, f_list in zip(times, freqs):
        for f in f_list:
            if f == 0:
                continue
            t_unrolled.append(t)
            f_unrolled.append(f)
    return t_unrolled, f_unrolled