code: def compute_trajectories(self):
          
          self.B = np.empty((self.T, self.N), 'int')
          self.B[-1, :] = self.A[-1]
          for t in reversed(range(self.T - 1)):
              self.B[t, :] = self.A[t + 1][self.B[t + 1]]
docs: Compute the N trajectories that constitute the current genealogy.
  
          Compute and add attribute ``B`` to ``self`` where ``B`` is an array
          such that ``B[t,n]`` is the index of ancestor at time t of particle X_T^n,
          where T is the current length of history.