code: def tracebacks(score_matrix, traceback_matrix, idx):
      
      return filter(lambda tb: tb != (),
                    _tracebacks(score_matrix,
                                traceback_matrix,
                                idx))
docs: Calculate the tracebacks for `traceback_matrix` starting at index `idx`.
  
      Returns: An iterable of tracebacks where each traceback is sequence of
        (index, direction) tuples. Each `index` is an index into
        `traceback_matrix`. `direction` indicates the direction into which the
        traceback "entered" the index.