def update_words(current_words, new_words):
    """ Determine words to add and delete for words update.

        Parameters:

            list. Currently existing words
            list. Words after update

        Return:

            list. Words to keep
            list. Words to remove
            list. Words to add
    """
    to_keep = [item for item in current_words for x in new_words if x in item]
    to_delete = [item for item in current_words if item[1] not in new_words]
    curr = [item[1] for item in current_words]
    to_add = [item for item in new_words if item not in curr]
    # Alternative method using a dictionary
    # to_add2 = [item for item in new_words if item not in dict(current_words).values()]
    return to_keep, to_delete, to_add