GtkMenuPositionFunc

[ken steen <ksteen earthlink net>]

I am writting an app with keyboard controls and would like a popup menu
that uses
x and y coordinates instead of the mouse positon.  In the gtk_menu_popup()
one of the arguments is GtkMenuPositionFunc which seems to do what I want. 
However, I can't figure out how to use it.  Can someone point me to an
example or code snippet.
Thanks,
Ken