[gentoo-user] How do I change an X keyboard layout?

[Alan Mackenzie]

Hello, Gentoo.

The two keyboard layouts I use in XFCE are both fine and dandy, but they
are incomplete.  In particular, I want the key combination
<Ctrl><Alt><shift><F1> to take me to tty13 in the same way that
<Ctrl><Alt><F1> takes me to tty1.

I've been searching for _hours_ trying to find out how to do this.  I
cannot find the keyboard layouts anywhere under /usr/share, where one
might expect them.  I can't find any relevant programs to manipulate
these data files with, even if I could find them.

Would somebody help me please.  Where are the X keyboard layouts stored,
and what program to I need to enhance them?

TIA!

-- 
Alan Mackenzie (Nuremberg, Germany).