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

        ngsit = 0
        gsrrs = np.inf
        while gsrrs > self.opt['GSTol'] and ngsit < self.opt['MaxGSIter']:
            self.X = self.GaussSeidelStep(self.S, self.X,
                                          self.cnst_AT(self.Y-self.U),
                                          self.rho, self.lcw, self.Wdf2)
            gsrrs = sl.rrs(
                self.rho*self.cnst_AT(self.cnst_A(self.X)) +
                self.Wdf2*self.X, self.Wdf2*self.S +
                self.rho*self.cnst_AT(self.Y - self.U))
            ngsit += 1

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