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对于两个命题，如果一个命题的条件核结论分别是另一个命题的结论和条件，那么这两个命题叫做互逆命题，其中一个命题叫做原命题，另一个命题叫做原命题的逆命题；
如果两个命题其中一个命题的条件和结论分别是另一个命题的条件和结论的否定，则这两个命题称互为否命题。
We can form some new conditional statements starting with a conditional statement p->q. In particular, there are three related conditional statements that occur so often that they have special names. The proposition q->p is called the converse of p->q. The contrapositive of p->q is the proposition ¬q->¬p. The proposition ¬p->¬q is called the inverse of p->q.