I am porting an old text-based system that runs on a
console to X11 with MWM (Lesstif) as the window manager.
F2 was recognised before the port but now it is ignored by
the application. I've been reading a bit about keyboard
mapping, but I really don't have enough knowledge on
keymappings to know how to approach this issue. Is there
some literature on this subject that might open my eyes to
a solution? Maybe even some good examples on how to use
xmodmap?

Problem is solved. Found a glich in my debugging method and
once corrected, I found that I didn't need to do anything
outside of my app to remap the function keys that had stopped
working.