code: def xstep(self):
          r
  
          YU = self.Y - self.U
          self.X = np.asarray(sl.cho_solve_ATAI(
              self.D, 1.0, self.block_sep0(YU) +
              self.D.T.dot(self.block_sep1(YU)), self.lu, self.piv),
              dtype=self.dtype)
docs: r"""Minimise Augmented Lagrangian with respect to
          :math:`\mathbf{x}`.