很难跟踪问题，因为宏的输入和输出是隐式的，不是显式的。
Difficulty tracing problems because the inputs and the outputs of the macros are implicit, not explicit.
宏流(macro - flow)的补偿服务可以显式地或者隐式地触发，而微流的补偿服务只能隐式地触发。
A macro-flow's compensation service can be triggered either explicitly or implicitly, while a micro-flow's compensation service can only be triggered implicitly.
如果在首次包含本库头文件之前用户未显式定义该宏，则缺省为20。
Defaults to 20 if not explicitly defined by the user before inclusion of the first library header.
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