comparing --get-selections and --set-selections

[Nick Cabatoff <ncc@cs.mcgill.ca>]

I'd like to have a tool which, given an input file I passed to dpkg
--set-selections and the output O from a subsequent call to dpkg 
--get-selections:

- warns me about every package listed in I and not in O (and perhaps
  tells me whether or not it exists in the apt lists? not certain if
  that's redundant or not; if it is, then this is obviously trivial)

- can tell me every package that exists in the O and not in I, and
  whether or not that package is a dependancy of a package that is in I

I'm not expecting a tools like this to already exist, but if so, great!
If not, can anyone recommend how to approach this in perl?  I'm hoping
there's some code out there already for studying package depenancies (and
possibly looking at the apt cache, if that should prove necessary).
Actually, I'm aware of some (in pkg-order), I just want to know if
there's anything more canonical/standard, or just if there are
alternatives.

---

While I'm on the subject, it seems to me that apt-get (0.3.11) doesn't
purge packages flagged in that state via --set-selections in some
circumstances.  My current theory is that it may have to do with the
package not existing in the lists files (only in status); can anyone
confirm or deny this?