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http://boole.cs.iastate.edu/book/3-%CA%B7(%C0%FA%CA%B7)/9-%CE%B4%B7%D6%C0%E0/yuedu/%D1%A7%C9%FA%B6%C1%CE%EF/kuangchongyuedu/xiandaiwenxuejingxuan/xiandaixinshixuan/beidao.htm