'Change a transfer function from the variable `z` to `z**-1`.
Parameters
num, den: 1d array_like
Sequences representing the coefficients of the numerator and
denominator polynomials, in order of ascending degree of \'z**-1\'.
That is, ``5 + 3 z**-1 + 2 z**-2`` is presented as ``[5, 3, 2]``.
Returns
num, den: 1d array_like
Sequences representing the coefficients of the numerator and
denominator polynomials, in order of descending degree of \'z\'.
That is, ``5z**2 + 3z + 2`` is presented as ``[5, 3, 2]``.'

@staticmethod
def _zinv_to_z(num, den):

    diff = (len(num) - len(den))
    if (diff > 0):
        den = np.hstack((den, np.zeros(diff)))
    elif (diff < 0):
        num = np.hstack((num, np.zeros((- diff))))
    return (num, den)
