code: def update(self):
          
          pars = self.subpars.pars
          responses = pars.control.responses
          fluxes = pars.model.sequences.fluxes
          self(len(responses))
          fluxes.qpin.shape = self.value
          fluxes.qpout.shape = self.value
          fluxes.qma.shape = self.value
          fluxes.qar.shape = self.value
docs: Determine the number of response functions.
  
          >>> from hydpy.models.arma import *
          >>> parameterstep('1d')
          >>> responses(((1., 2.), (1.,)), th_3=((1.,), (1., 2., 3.)))
          >>> derived.nmb.update()
          >>> derived.nmb
          nmb(2)
  
          Note that updating parameter `nmb` sets the shape of the flux
          sequences |QPIn|, |QPOut|, |QMA|, and |QAR| automatically.
  
          >>> fluxes.qpin
          qpin(nan, nan)
          >>> fluxes.qpout
          qpout(nan, nan)
          >>> fluxes.qma
          qma(nan, nan)
          >>> fluxes.qar
          qar(nan, nan)