Currently we have no way to specify the last layout index when using the group actions.

This is an issue for using e.g. ISO_Last_Group. It current compat definition is:

    interpret ISO_Last_Group {
	action= LockGroup(group=2);
    };

What about:

    interpret ISO_Last_Group {
	action= LockGroup(group=last);
    };

where last denotes the last group index, provided by the RMLVO API. It would allow to use also expressions, e.g. last - 1, etc.

The issue is that if using the string API, we cannot not know the total number of groups until parsing all the keymap, contrary to the RMLVO interface where we know it after resolving the rules, because with the latter API we enforce a single group per xkb_symbols section.

I would rather avoid any solution that implies keeping an AST around.

It would be nice to have the mirrored first index, which would be fixed to the expected group 1 (1-indexed).