Occurences of comma-separated values

For instance, I have the following XML-File:

<list>
  <item1 ids="a,b,c,e" />
  <item2 ids="b,c,d" />
  <item3 ids="a,c,d,e" />
  <item4 ids="e,f" />
  <item5 ids="a,c,d,e,g" />
</list>

I contemplate how to count the occurences of
(a) all ids (e.g. 'a') in the document and
(b) to count the occurences of a specific (known) id (e.g. 'f') in the document.

For case a the output should be like this:

a: 3
b: 2
c: 4
d: 3
e: 4
f: 1
g: 1

I started to learn XSL yesterday. For several other problems, I found an easily adaptable solution on the web, numerous time on this list here.
But for my current problem, there was no such help available.
Of course, I found here a similar posting which deals with occurences of equal nodes (http://www.stylusstudio.com/xsllist/200309/post20830.html), but I don't know how to adapt it. So please find a solution and help me and others who have the same problem.


Thanks in advance,

S. Renault
-- 


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