常量和宏的命名一般是大写。
Names of macros and constants are conventionally in uppercase.
原因在于 Scheme 会对 pi 和 e 进行重命名，使它们不会与包含或嵌入宏的范围中的其他名称冲突。
The reason is that, as mentioned earlier, Scheme will rename pi and e so they don't conflict with other names in enclosing or nested scopes.
这意味着宏所使用的所有临时变量都会在替换发生之前自动重新进行命名，从而防止名字产生冲突。
This means that all temporary variables used by a macro are automatically renamed before the substitution occurs in order to prevent them from having conflicting names.
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