Can you use RTTI to determine a union member?

[Janice Caron]

On 10/16/07, Christian Kamm <kamm.incasoftware at shift-at-left-and-remove-this.de
> If A, B, C are classes, you could also check with
>
> u.a.classinfo is A.classinfo
> u.a.classinfo is B.classinfo
> u.a.classinfo is C.classinfo.

That's really cool, but I can't find any documentation on .classinfo.
I looked on the DigitalMars site in the pages "Classes", "Expressions"
and "Properties", and after that I got stuck. Where is this
documented?