Line: the abstraction symbol of the neighboring relationship between two neighboring countries. This line can not be crossed or covered except on the two endpoints.
相邻线段：两个国家之间相邻关系的抽象化代表符号，连接具有相邻关系的两个国家点的线段。
Line: the abstraction symbol of the neighboring relationship between two neighboring countries. This line can not be crossed or covered except on the two endpoints.
相邻线段：两个国家之间相邻关系的抽象化代表符号，连接具有相邻关系的两个国家点的线段。
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