What should postrm purge actually do?

[Richard Kettlewell <rjk@greenend.org.uk>]

Is it written down anywhere what postrm purge is supposed to do? 
Presumably remove some set of files, but what criteria should be used to 
choose which?

The policy document is not much help; s6.8 says when it is called, but 
not what it actually needs to do.  I can't find more detail, though of 
course I may have missed it.

Things I think it should remove:
   - generated configuration files

Things I'm uncertain about, but that wouldn't be missed:
   - infrastructural stuff (lockfiles, sockets, etc)
   - files containing cached data

Things I'm uncertain about, that someone might actually miss:
   - log files
   - data accumulated from users

Configuration files might be missed too, so obviously --purge isn't 
intended to be nondestructive, the question is how destructive is it 
supposed to be and to what extent is it responsible for tidying up.

ttfn/rjk