A more real-world example is TeX, which I use directly for typesetting,
  and which is used by doxygen.  If I remove doxygen, I *certainly* do *not*
  want TeX to be removed.

I've wanted this too, even if I just ask the system for a list of
packages that aren't marked as wanted and have no depending packages
and then think.  While meta-packages address this, they're awkward in
a binary-only environment.

If we add the ability to set variables on installed packages
(putprop/getprop, basically), then we could also use this to mark
packages as having had a dependency replaced.  Specifically, 'make
replace' would set 'unsafe_dependency' to 'yes' on every package that
depended on the one being replaced.  Then, one could find the set of
such packages, tsort them, run 'make replace' on the first, and then
repeat the fetch/tsort.  This would have the same end result as make
update, but with different, perhaps more desirable brokenness: rather
than missing packages due to a failed build, some packages might not
work due to dependency ABI changes.


-- 
        Greg Troxel <gdt@ir.bbn.com>