docs: def _traceback_to_alignment(tb, a, b):
      """Convert a traceback (i.e. as returned by `tracebacks()`) into an alignment
      (i.e. as returned by `align`).
  
      Arguments:
        tb: A traceback.
        a: the sequence defining the rows in the traceback matrix.
        b: the sequence defining the columns in the traceback matrix.
  
      Returns: An iterable of (index, index) tupless where ether (but not both)
        tuples can be `None`.
      """
      # We subtract 1 from the indices here because we're translating from the
      # alignment matrix space (which has one extra row and column) to the space
      # of the input sequences.
      for idx, direction in tb:
          if direction == Direction.DIAG:
              yield (idx[0] - 1, idx[1] - 1)
          elif direction == Direction.UP:
              yield (idx[0] - 1, None)
          elif direction == Direction.LEFT:
              yield (None, idx[1] - 1)