def pad_sentence(sentence, window_len, lsl, rsl, mask):
    """
    :param sentence: Sentence to pad
    :param window_len: The ammount of tokens in the desired sentence
    :param lsl: (left-side length) The current token count to the left of the central token
    :param rsl: (right-side length) The current token count to the right of the central token
    :param mask: The masking token
    :return: A sentence padded with BOS, EOS and MASK tokens
    """

    for i in range(window_len - lsl):
        sentence.insert(0, mask)

    for i in range(window_len - rsl):
        sentence.append(mask)

    return sentence