def xstep(self):
        r"""Minimise Augmented Lagrangian with respect to
        :math:`\mathbf{x}`.
        """

        ngsit = 0
        gsrrs = np.inf
        YU = self.Y - self.U
        SYU = self.S + YU[..., -1]
        YU[..., -1] = 0.0
        ATYU = self.cnst_AT(YU)
        while gsrrs > self.opt['GSTol'] and ngsit < self.opt['MaxGSIter']:
            self.X = self.GaussSeidelStep(
                SYU, self.X, ATYU, 1.0, self.lcw, 1.0)
            gsrrs = sl.rrs(
                self.cnst_AT(self.cnst_A(self.X)),
                self.cnst_AT(self.cnst_c() - self.cnst_B(self.Y) - self.U)
            )
            ngsit += 1

        self.xs = (ngsit, gsrrs)
r"""Minimise Augmented Lagrangian with respect to
        :math:`\mathbf{x}`.