#include header guard format?

Question

I know it makes little difference to a project but, assuming you use #defined header guards for your C++ code, what format do you use? e.g. assuming a header called f

Answer 1

I use

_

or

_INCLUDED_

Keeping it synchronous with folder hierarchies is too annoying (friend of refactoring), GUIDs are too annoying, the date suffix is good enough. If I would have to equally named files on the same day, I would

_a
_b
_...