'Calculates an absolute difference value. 0 means no difference,
i.e. the count vectors are identical.
:param other: The CountVector to calculate the difference to.
:return:      An absolute difference value.'

def difference(self, other):

    assert isinstance(other, CountVector)
    assert (len(other) == len(self))
    return sqrt(sum((((x - y) ** 2) for (x, y) in zip(self, other))))
