有两种形式，第一种是宏，它就像是具有延迟参数的方法调用一样，这些参数可用特殊的方式计算出来。

They take two forms, the first one is macros, which is basically like method calls with lazy arguments that can be evaluated in special ways.

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在这个宏执行时，它按照以下形式将my - if表达式与模板进行匹配(换句话说，将宏调用与宏定义模式进行匹配)。

When this macro executes, it will match the my-if expression up to the template like this (in other words, matching a macro invocation to a macro definition pattern).

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