code: def update(self):
          
          responses = self.subpars.pars.control.responses
          self.shape = len(responses)
          self(responses.ar_orders)
docs: Determine the total number of AR coefficients.
  
          >>> from hydpy.models.arma import *
          >>> parameterstep('1d')
          >>> responses(((1., 2.), (1.,)), th_3=((1.,), (1., 2., 3.)))
          >>> derived.ar_order.update()
          >>> derived.ar_order
          ar_order(2, 1)