Freeing up XFCE key bindings. How do I do this?

[Alan Mackenzie]

Hello, XFCE.

>From time to time I run Emacs in XFCE.  This is XFCE running on Gentoo
GNU/Linux on amd_64 hardware.

Unfortunately, on depressing a key <Fn>, for 1 <= n <= 12, the keypress
gets intercepted by XFCE rather than being passed through to Emacs.

I would like these function keys to be passed through to Emacs.  How do
I arrange this?  Where is the documentation describing how to do this?

Thanks in advance!

-- 
Alan Mackenzie (Nuremberg, Germany).