'construct AR and MA polynomials that are zero-padded to a common length
Parameters
maxlag : int
new length of lag-polynomials
Returns
ar : ndarray
extended AR polynomial coefficients
ma : ndarray
extended AR polynomial coefficients'

def pad(self, maxlag):

    arpad = np.r_[(self.ar, np.zeros((maxlag - self.nar)))]
    mapad = np.r_[(self.ma, np.zeros((maxlag - self.nma)))]
    return (arpad, mapad)
