code: def xistep(self, i):
          r
  
          self.YU[:] = self.Y - self.U[..., i]
          b = np.take(self.ZSf, [i], axis=self.cri.axisK) + \
              self.rho*sl.rfftn(self.YU, None, self.cri.axisN)
  
          self.Xf[..., i] = sl.solvedbi_sm(np.take(
              self.Zf, [i], axis=self.cri.axisK),
                                           self.rho, b, axis=self.cri.axisM)
          self.X[..., i] = sl.irfftn(self.Xf[..., i], self.cri.Nv,
                                     self.cri.axisN)
docs: r"""Minimise Augmented Lagrangian with respect to :math:`\mathbf{x}`
          component :math:`\mathbf{x}_i`.