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

        self.X, self.ss = sp.prox_nuclear(self.S - self.Y - self.U,
                                          1/self.rho)
r"""Minimise Augmented Lagrangian with respect to
        :math:`\mathbf{x}`.