Don't:

	MACRO(
		a,
	#if FOO
		b,
	#else
		c,
	#endif
		d
	)

A function like macro was invoked whose arguments extended for
multiple lines, which included preprocessor statements. This is
almost certainly an error brought about by a missing right parenthesis.

By the rules of Standard C the preprocessing directive is absorbed
into the macro argument but then will not subsequently get executed.
For this reason some compilers treat the apparent preprocessor
directive as a directive. This is logical but not portable.

It is therefore best to avoid this construct.