emerge --depclean

 * Always study the list of packages to be cleaned for any obvious
 * mistakes. Packages that are part of the world set will always
 * be kept.  They can be manually added to this set with
 * `emerge --noreplace <atom>`.  Packages that are listed in
 * package.provided (see portage(5)) will be removed by
 * depclean, even if they are part of the world set.
 *
 * As a safety measure, depclean will not remove any packages
 * unless *all* required dependencies have been resolved.  As a
 * consequence of this, it often becomes necessary to run
 * `emerge --update --newuse --deep @world` prior to depclean.

Calculating dependencies... done!
 * Dependencies could not be completely resolved due to
 * the following required packages not being installed:
 *
 *   sys-apps/systemd:0= pulled in by:
 *     sys-apps/dbus-1.8.10
 *
 *   sys-apps/systemd:0/2= pulled in by:
 *     sys-apps/dbus-1.8.10
 *
 * Have you forgotten to do a complete update prior to depclean? The
 * most comprehensive command for this purpose is as follows:
 *
 *   emerge --update --newuse --deep --with-bdeps=y @world
 *
 * Note that the --with-bdeps=y option is not required in many
 * situations. Refer to the emerge manual page (run `man emerge`)
 * for more information about --with-bdeps.
 *
 * Also, note that it may be necessary to manually uninstall
 * packages that no longer exist in the portage tree, since it may
 * not be possible to satisfy their dependencies.