Caching variables during completion

[Jan Eike von Seggern <jan.eike.von.seggern@desy.de>]

Hello,

my question first: Is it possible to set a variable only once during 
every command-line completion, i.e. only after hitting <Tab> for the 
first time and then keeping the contents for this specific command-line?

Explanation:
I'm trying to improve the completion for xrandr (mainly screen-scraping 
the available outputs and associated modes). However, my first try 
involved screen-scraping the output of `xrandr -q` every time a 
completion was made, which was slow. I then resorted to cache the 
available output/modes in a global variable. But this fails if a new 
output gets available.

Best,
Eike