| Traditional miscellany

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Here are some things to be aware of when using the traditional preprocessor.

- Preprocessing directives are recognized only when their leading `#` appears in the first column. There can be no whitespace between the beginning of the line and the `#`, but whitespace can follow the `#`.
- A true traditional C preprocessor does not recognize `#error` or `#pragma`, and may not recognize `#elif`. CPP supports all the directives in traditional mode that it supports in ISO mode, including extensions, with the exception that the effects of `#pragma GCC poison` are undefined.
- __STDC__ is not defined.
- If you use digraphs the behavior is undefined.
- If a line that looks like a directive appears within macro arguments, the behavior is undefined.