$ emerge -1vDtu @world --with-bdeps=y --quiet-build=n --exclude em8300-libraries emerge: the given set 'world' contains a non-existent set named 'mg_xfce-mini'. $ emerge -C @mg_xfce-mini emerge: There are no sets to satisfy 'mg_xfce-mini'. It would be great if portage suggested a way to remove the set (i.e. 'try emaint world' or something like that).
This is fixed in 2.1.11.5 and 2.2.0_alpha116: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=f736935de2bb1370524302d9cfe915c16d922bb9