code: def xstep(self):
          r
  
          self.YU[:] = self.Y - self.U
          self.block_sep0(self.YU)[:] += self.S
          YUf = sl.rfftn(self.YU, None, self.cri.axisN)
          b = sl.inner(np.conj(self.Zf), self.block_sep0(YUf),
                       axis=self.cri.axisK) + self.block_sep1(YUf)
  
          self.Xf[:] = sl.solvemdbi_ism(self.Zf, 1.0, b, self.cri.axisM,
                                        self.cri.axisK)
          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}`.