'Construct a new empty conditional frequency distribution.  In
particular, the count for every sample, under every condition,
is zero.
:param cond_samples: The samples to initialize the conditional
frequency distribution with
:type cond_samples: Sequence of (condition, sample) tuples'

def __init__(self, cond_samples=None):

    defaultdict.__init__(self, FreqDist)
    if cond_samples:
        for (cond, sample) in cond_samples:
            self[cond][sample] += 1
