code: def xstep(self):
          r
  
          self.X = np.asarray(sl.lu_solve_AATI(self.Z, self.rho, self.SZT +
                              self.rho*(self.Y - self.U), self.lu, self.piv,),
                              dtype=self.dtype)
docs: r"""Minimise Augmented Lagrangian with respect to
          :math:`\mathbf{x}`.