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(m4.info)Conditionals

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Conditionals, loops and recursion
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   Macros, expanding to plain text, perhaps with arguments, are not
quite enough.  We would like to have macros expand to different things,
based on decisions taken at run-time.  E.g., we need some kind of
conditionals.  Also, we would like to have some kind of loop construct,
so we could do something a number of times, or while some condition is
true.

Ifdef

Testing if a macro is defined

Ifelse

If-else construct, or multibranch

Loops

Loops and recursion in m4