code: def calc_next_ma_coef(self, ma_order, ma_model):
          
          idx = ma_order-1
          coef = ma_model.coefs[idx]
          for jdx, ar_coef in enumerate(self.ar_coefs):
              zdx = idx-jdx-1
              if zdx >= 0:
                  coef -= ar_coef*ma_model.coefs[zdx]
          self.ma_coefs = numpy.concatenate((self.ma_coefs, [coef]))
docs: Determine the MA coefficients of the ARMA model based on its
          predetermined AR coefficients and the MA ordinates of the given
          |MA| model.
  
          The MA coefficients are determined one at a time, beginning with the
          first one.  Each ARMA MA coefficient in set in a manner that allows
          for the exact reproduction of the equivalent pure MA coefficient with
          all relevant ARMA coefficients.