def _get_freqs(*series: Union[TimeSeries, None]):
    """
    Returns list with the frequency of all specified (i.e. non-`None`) `series`.
    """
    freqs = []
    for ts in series:
        if ts is not None:
            freqs.append(ts.freq)
    return freqs

    Returns list with the frequency of all specified (i.e. non-`None`) `series`.
    
darts/utils/data/tabularization/tabularization.py