code: def update(self):
          
          pars = self.subpars.pars
          coefs = pars.control.responses.ma_coefs
          self.shape = coefs.shape
          self(coefs)
          pars.model.sequences.logs.login.shape = self.shape
docs: Determine all MA coefficients.
  
          >>> from hydpy.models.arma import *
          >>> parameterstep('1d')
          >>> responses(((1., 2.), (1.,)), th_3=((1.,), (1., 2., 3.)))
          >>> derived.ma_coefs.update()
          >>> derived.ma_coefs
          ma_coefs([[1.0, nan, nan],
                    [1.0, 2.0, 3.0]])
  
          Note that updating parameter `ar_coefs` sets the shape of the log
          sequence |LogIn| automatically.
  
          >>> logs.login
          login([[nan, nan, nan],
                 [nan, nan, nan]])