由于这一层关系，它操作起来更像是一个用户定义的关系操作符。
With the correlation it operates more like a user-defined relational operator.
前面的示例创建两个 dictionary 并使用它们测试 < 关系操作符。
The previous example creates two dictionaries and uses them to test the < relational operator.
对称的操作符，如算术操作符、相等操作符、关系操作符和位操作符，最好定义为普通非成员函数。
Symmetric operators, such as the arithmetic, equality, relational, and bitwise operators, are best defined as ordinary nonmember functions.
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