'Hampel\'s function is defined piecewise over the range of z'

def _subset(self, z):

    z = np.fabs(np.asarray(z))
    t1 = np.less_equal(z, self.a)
    t2 = (np.less_equal(z, self.b) * np.greater(z, self.a))
    t3 = (np.less_equal(z, self.c) * np.greater(z, self.b))
    return (t1, t2, t3)
