def get_substitution(rules, from_list):
    """
    This function will choose a single subject word that hasn't exist in the replacement rule dict we already have.
    :param rules: Current replacement dict.
    :param from_list: The substitution alternatives.
    :return: A single subject word that is not in the current replacement dict.
    """
    for i in from_list:
        if i not in rules.values():
            return i