def add_start_end(tokens, start_word="<START>", end_word="<END>"):
    """ Add  start and end words for a caption string

    Args:
      tokens: original tokenized caption
      start_word: word to indicate start of a caption sentence
      end_word: word to indicate end of a caption sentence
    Returns:
      token_caption: tokenized caption
    """
    token_caption = [start_word]
    token_caption.extend(tokens)
    token_caption.append(end_word)

    return token_caption