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JOIN(x, y) calls JOIN_AGAIN(x, y) so that, if x or y is a macro, they are it is expanded before the ## operator pastes them together.
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The macro invocation str(foo) expands to "foo".
Compliant Solution
To stringify the result of expansion of a macro argument, you must use two levels of macros:
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The macro invocation xstr(foo) expands to "4". This is because 's' is stringified when it is used in str(), so it is not macro expanded first. However, 's' is an ordinary argument to xstr(), so it is completely macro expanded before xstr() is expanded. Consequently, by the time str() gets to its argument, it has already been macro expanded.
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