Information on xkb group index in bits 13 & 14 of event state information for input events

[Farblos]

Hi Xorg,

in this FVWM issue:

  https://github.com/fvwmorg/fvwm3/issues/861

bits 13 and 14 encoding the XKB group index in event->xkey.state
on behalf of the XKB extension interfere with FVWM's modifier
handling.

At least that is what I have understood so far from my tests and

  https://www.x.org/releases/X11R7.6/doc/libX11/specs/XKB/xkblib.html#xkb_state_to_core_protocol_state_transformation

Is there anything better than just masking out bits 13 and 14 in
event->xkey.state as I have proposed as solution for that issue?
Is there some constant that represents these two bits?

I have tried searching the Xorg sources and the web but couldn't
find anything more enlighting than above section in the XKB
library specification.

Thanks