I would like to run # emerge @preserved-rebuild --usepkg-exclude=@preserved-rebuild -vbka during system updates, but sets are now not supported in exclude options. I run 'emerge --depclean --with-bdeps=n -qa' after every update, so i have no build deps installed, rebuilding them every time is pointless. Also supporting sets in other *exclude* and *ignore* options could be useful.
The set expansion should be delayed until depgraph.select_files() is called, so that the spinner is responsive during set expansion, taking advantage of this code from bug #461412: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=8e130a60c9f4d6ab7f1de547015c4ce0b0045041
