'Return the cumulative frequencies of the specified samples.
If no samples are specified, all counts are returned, starting
with the largest.
:param samples: the samples whose frequencies should be returned.
:type samples: any
:rtype: list(float)'

def _cumulative_frequencies(self, samples):

    cf = 0.0
    for sample in samples:
        cf += self[sample]
        (yield cf)
