code: def ystep(self):
          r
  
          rho = self.Nb * self.rho
          mAXU = np.mean(self.AX + self.U, axis=-1)
          self.Y[:] = self.prox_g(mAXU, rho)
docs: r"""Minimise Augmented Lagrangian with respect to :math:`\mathbf{y}`.