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This file is a workaround for the issue that if you say `a ## b` to
create a token that is the name of a macro, the C preprocessor won't
expand that macro. So you can't say this:
#define FOO__SQUARE(x) ((x)*(x))
#define FOO__CUBE(x) ((x)*(x)*(x))
#define FOO(func, x) FOO__##func(x)
Instead, the standard C trick is to add a layer of indirection:
#define PASTE(a,b) PASTE__(a,b)
#define PASTE__(a,b) a ## b
#define FOO__SQUARE(x) ((x)*(x))
#define FOO__CUBE(x) ((x)*(x)*(x))
#define FOO(func, x) PASTE(FOO__, func)(x)
We should use this kind of trick sparingly, since it gets confusing.
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