You'd have to call it something like `_Fn` instead, or you're going to conflict with existing code (the C standard reserves identifiers starting with an undersc

You'd have to call it something like `_Fn` instead, or you're going to conflict with existing code (the C standard reserves identifiers starting with an underscore followed by a capital letter; it also reserves identifiers starting with two underscores, but it's fairly common for code to ignore that and use identifiers like that anyway).