O(N) version of two-filter smoothing.

        This method should not be called directly, see twofilter_smoothing.

def _twofilter_smoothing_ON(self, t, ti, info, phi, lwinfo, return_ess,
                               modif_forward, modif_info):
        """O(N) version of two-filter smoothing.

        This method should not be called directly, see twofilter_smoothing.
        """
        if modif_info is not None:
            lwinfo += modif_info
        Winfo = rs.exp_and_normalise(lwinfo)
        I = rs.multinomial(Winfo)
        if modif_forward is not None:
            lw = self.wgt[t].lw + modif_forward
            W = rs.exp_and_normalise(lw)
        else:
            W = self.wgt[t].W
        J = rs.multinomial(W)
        log_omega = self.model.logpt(t + 1, self.X[t][J], info.hist.X[ti][I])
        if modif_forward is not None:
            log_omega -= modif_forward[J]
        if modif_info is not None:
            log_omega -= modif_info[I]
        Om = rs.exp_and_normalise(log_omega)
        est = np.average(phi(self.X[t][J], info.hist.X[ti][I]), axis=0,
                         weights=Om)
        if return_ess:
            return (est, 1. / np.sum(Om**2))
        else:
            return est