code: def xstep_check(self, b):
          r
  
          if self.opt['LinSolveCheck']:
              Zop = lambda x: sl.inner(self.Zf, x, axis=self.cri.axisM)
              ZHop = lambda x: sl.inner(np.conj(self.Zf), x,
                                        axis=self.cri.axisK)
              ax = ZHop(Zop(self.Xf)) + self.Xf
              self.xrrs = sl.rrs(ax, b)
          else:
              self.xrrs = None
docs: r"""Check the minimisation of the Augmented Lagrangian with
          respect to :math:`\mathbf{x}` by method `xstep` defined in
          derived classes. This method should be called at the end of any
          `xstep` method.