emerge --depclean

dev-lang/nasm
    selected: 2.09.10
   protected: none
     omitted: none
 dev-python/numpy
    selected: 1.6.1-r1
   protected: none
     omitted: none
 dev-python/pygobject
    selected: 2.28.6-r51
   protected: none
     omitted: none
 virtual/editor
    selected: 0
   protected: none
     omitted: none

lupo@gentoo-laptop ~ % equery d dev-lang/nasm
 * These packages depend on dev-lang/nasm:
media-libs/flac-1.2.1-r4 (x86 ? dev-lang/nasm)
media-libs/libjpeg-turbo-1.1.90_p20120129 (amd64 ? dev-lang/nasm)
                                          (x86 ? dev-lang/nasm)
                                          (amd64-linux ? dev-lang/nasm)
                                          (x86-linux ? dev-lang/nasm)
media-libs/libsdl-1.2.15 (x86 ? >=dev-lang/nasm-0.98.39-r3)
media-libs/xvid-1.3.2 (amd64 ? >=dev-lang/nasm-2)
                      (x86 ? >=dev-lang/nasm-2)
                      (x86-fbsd ? >=dev-lang/nasm-2)
media-sound/lame-3.99.4 (mmx ? dev-lang/nasm)

quickpkg --include-config=y dev-lang/nasm

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, it is often 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:
 * 
 *   dev-python/pygobject:3 pulled in by:
 *     dev-python/pygobject-2.28.6-r51
 * 
 * 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.
 * IMPORTANT: 2 news items need reading for repository 'gentoo'.
 * Use eselect news to read news items.

[ebuild  N     ] dev-python/pycairo-1.10.0-r2  USE="svg -doc -examples -test" 0 kB
[ebuild  NS    ] dev-python/pygobject-3.0.3 [2.28.6-r51] USE="cairo threads -examples -test" 0 kB

emerge -puDNv @world