后置条件，英文名称post-condition，意思是指输出定义域上的谓词的布尔值。
合同包括不变量，先决条件和后置条件。
The contract includes invariants, preconditions, and post conditions.
后置条件： 执行外部组件后的预期结果。
Post-conditions: Expected results after the execution of the external component.
后置条件测试通常用方法返回值的断言来表示。
Postconditions are often expressed as assertions about method return values.
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