说明：  若结构是非空有限集，则有且仅有一个开始结点和一个终端结点，并且所有结点都最多只有一个直接前趋和一个直接后继。 →可表示为：（a1 , a2 , ……, an）
(If the structure of non-empty finite set, there are only a beginning and node and a terminal node, and all nodes are at most before becoming a direct and a direct follow-up. → can be expressed as: (a1, a2, ... ..., an))