'Returns a vector containing the variances of the discrete
distributions.
A vector is returned containing the variance for each row of
`xk`, using the probabilities in the corresponding row of
`pk`.'

def var(self):

    mn = self.mean()
    xkc = (self.xk - mn[:, None])
    return (self.pk * ((self.xk - xkc) ** 2)).sum(1)
