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

        self.Y = np.asarray(sp.prox_l1(self.AX + self.U,
                                       (self.lmbda / self.rho) * self.wl1),
                            dtype=self.dtype)
        super(BPDN, self).ystep()
r"""Minimise Augmented Lagrangian with respect to
        :math:`\mathbf{y}`.