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A conditional in the C preprocessor begins with a conditional
directive: `#if', `#ifdef' or `#ifndef'.
See section Conditionals and Macros, for information on `#ifdef' and
`#ifndef'; only `#if' is explained here.
- If: Basic conditionals using `#if' and `#endif'.
- Else: Including some text if the condition fails.
- Elif: Testing several alternative possibilities.
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