Extract a single trajectory from the particle history.

        The final state is chosen randomly, then the corresponding trajectory 
        is constructed backwards, until time t=0.

def extract_one_trajectory(self):
        """Extract a single trajectory from the particle history.

        The final state is chosen randomly, then the corresponding trajectory 
        is constructed backwards, until time t=0. 
        """
        traj = []
        for t in reversed(range(self.T)):
            if t == self.T - 1:
                n = rs.multinomial_once(self.wgt[-1].W)
            else:
                n = self.A[t + 1][n]
            traj.append(self.X[t][n])
        return traj[::-1]