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

["Robert P. J. Day" <rpjday@mindspring.com>]

  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