它还有助于确定源和目标之间的关系类型，比如聚合、转换和算术运算。
It also helps in types of relationship between source and target, such as aggregations, transformations, and arithmetic operations.
算术级数：一个数量在其变化的过程中，每一次变化后的量是变化前的若干倍，也就是乘积的关系。
Arithmetic progression: a number of its change in the process, after each change in the amount of change several times before, that is the product of the relationship.
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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