The last one has never been used in practice yet ; the second one is used under strict conditions ; and the first one is used in every comer of the world . we should use the purchase method in all combinations 重新开始法在全球还没得到实际应用，权益集合法在少数国家(如美国)得到了有限的应用，购买法是应用最广的一种方法。
