emerge -C world

# emerge -avC world
emerge: the given set 'world' does not support unmerge operations

# emerge -avC `qlist -C -I`
 * This action can remove important packages! In order to be safer, use
 * `emerge -pv --depclean <atom>` to check for reverse dependencies before
 * removing packages.

>>> These are the packages that would be unmerged:
 * Not unmerging package dev-lang/python-2.7.1-r1 since there is no valid
 * reason for Portage to unmerge currently used Python interpreter.
 * Not unmerging package sys-apps/portage-2.1.9.50 since there is no valid
 * reason for Portage to unmerge itself.