def check_deps(sent):
    """
    Checks that each dependency link refer to an existing word id
    Args: list of tuples (ln, word objs)
    Return: list of tuples (ln, word objs) with false head
    """
    headless_words = []
    ids = [word.nid for ln, word in sent]
    for ln, word in sent:
        # on zappe les contractés
        if "-" in word.nid:
            continue
        if not (word.head in ids) and word.head != '0':
            headless_words.append((ln, word)) 
    return headless_words