Sorting Order of Completion Candidates

[Florian Beck]

While writing some code to index my notes I have run into a problem.
I have a carefully crafted list of keywords sorted in order of 
frequency. (When assigning new keywords I want to consider the most 
frequent keywords first.) However

> (completing-read "Prompt: " '("a" "c" "b"))

gives me

> Possible completions are:
> a
> b
> c

whereas I want


> Possible completions are:
> a
> c
> b

If there is no option or hook, if guess I could just modify the 
*Completions* buffer. Any hints how/where it is set up?

Many thanks in advance!