'Add counts from two ConditionalFreqDists.'

def __add__(self, other):

    if (not isinstance(other, ConditionalFreqDist)):
        return NotImplemented
    result = ConditionalFreqDist()
    for cond in self.conditions():
        newfreqdist = (self[cond] + other[cond])
        if newfreqdist:
            result[cond] = newfreqdist
    for cond in other.conditions():
        if (cond not in self.conditions()):
            for (elem, count) in other[cond].items():
                if (count > 0):
                    result[cond][elem] = count
    return result
