code: def xstep(self):
          r
  
          self.cgit = None
          self.YU[:] = self.Y - self.U
          b = self.ZSf + self.rho*sl.rfftn(self.YU, None, self.cri.axisN)
          self.Xf[:], cgit = sl.solvemdbi_cg(self.Zf, self.rho, b,
                                             self.cri.axisM, self.cri.axisK,
                                             self.opt['CG', 'StopTol'],
                                             self.opt['CG', 'MaxIter'], self.Xf)
          self.cgit = cgit
          self.X = sl.irfftn(self.Xf, self.cri.Nv, self.cri.axisN)
          self.xstep_check(b)
docs: r"""Minimise Augmented Lagrangian with respect to :math:`\mathbf{x}`.