DOCBOOK: how to represent key sequence "Ctrl Alt Fn"??

from what i've read, it appears that the officially sanctioned to 
represent something like the simultaneous key combination 
"Ctrl Alt Del" would be

  <keycombo action="simul">
   <keycap>Ctrl</keycap>
   <keycap>Alt</keycap>
   <keycap>Del</keycap>
  </keycombo>

and if i was planning on needing it frequently, i could easily
create an entity for it.

  but what if i wanted to similarly represent "Ctrl Alt Fn", where
the "n" part was replaceable?  is there any way to add parameters
to regular entity definitions?

  and, on a slightly related note, any pointers to publicly-available
sets of common, useful entity definitions for writing computer-related
documents?

rday