e speed?

    >>> harmonic_mean([40, 60])
    48.0

Suppose a car travels 40 km/hr for 5 km, and when traffic clears,
speeds-up to 60 km/hr for the remaining 30 km of the journey. What
is the average speed?

    >>> harmonic_mean([40, 60], weights=[5, 30])
    56.0

If ``data`` is empty, or any element is less than zero,
``harmonic_mean`` will raise ``StatisticsError``.
z.harmonic mean does not support negative valuesr@