code: def ystep(self):
          r
  
          AXU = self.AX + self.U
          self.Y[..., 0:-1] = sp.prox_l2(AXU[..., 0:-1], self.mu/self.rho)
          self.Y[..., -1] = sp.prox_l1(AXU[..., -1],
                                       (self.lmbda/self.rho) * self.Wl1)
docs: r"""Minimise Augmented Lagrangian with respect to
          :math:`\mathbf{y}`.