See https://blueprints.launchpad.net/xbps/+spec/essential-upgrades
for complete information.
Implementation used almost the same code than the pseudo-code explained
in the blueprint. Works really fine, tested with glibc-2.10.1 -> 2.11
upgrade.
--HG--
extra : convert_revision : xtraeme%40gmail.com-20091109013004-szdc9ngb3i70bc4p