Standardized predefined CPP symbols, down with folklore

[Geo Swan]

I agree with the need for standards.
Further, these standards should be properly documented.
I learned that cpp automatically sharp defined "unix",
when I tried to declare a variable named "unix".  (Try it sometime.)
When I posted a local gripe, a number of people kindly
explained this convention to me  (In fact, prior to posting
my gripe, I figured the convention out through trial and error.)

This is folklore.

None of my informants knew if and where this feature was documented.
Undocumented features like this are booby-traps.