def _extract_contractions(sent):
    """
    Args:
        sent - a single sentence split up into (word, pos) tuples.
    Returns:
        List with the indices in the sentence where the contraction
        starts.
        Or None if no contractions are in the sentence.

    Based on the POS-tags and the existence of an apostrophe or not,
    extract the existing contractions.
    """
    idx_lst = []
    for i, word_pos in enumerate(sent):
        # If the word in the word_pos tuple begins with an apostrophe,
        # add the index to idx_list.
        if word_pos[0][0] == "'":
            if word_pos[1] != 'POS':
                # POS stands for possessive pronoun
                idx_lst.append(i)
        elif word_pos[0] == "n't":
            # n't is treated extraordinarily and added explicitly
            idx_lst.append(i)
    if idx_lst:
        return idx_lst