loop invariant

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p,q都是断言 R就是循环不变式

— By their role with respect to the postcondition (Section 3.1), leading us to distinguishbetween “essential” and “bounding” invariant properties.
— By the transformation technique that yields the invariant from the postcondition (Sec-tion 3.2). Here we have techniques such as uncoupling and constant relaxation

loop invariant分类：

1. 按role分：逼近postcondition是越来越聚集or越爱越分散靠近bounding
2. 变量：解耦（改变变量）和常数（i=10）