emerge will sometimes suggest that the user invoke emerge -v @preserved-rebuild in order to clean up after changes in library names. This is a nice feature and works reasonably well. However, as a side effect, the set name gets added to the /var/lib/portage/world file, and I am told this is a violation of PMS. Indeed, once this happens, pkgcore complains about pkgcore.ebuild.errors.MalformedAtom: atom '@preserved-rebuild' is malformed: error @preserved-rebuild In fact, the man page for portage says that the world file is a list of packages, and I don't think that a set qualifies as a package.
This isn't a violation of PMS (for whatever reason, PMS doesn't cover world currently)- blame me since I did the verification after mentioning this to fmccor. Meanwhile, still after discussion of this since this pretty heavily changes the meaning of world while making it pissier for alt managers to try and play nice with portage.
As ferringb just mentioned the same in #gentoo-portage, we'll look into other options as well. However, the worldfile isn't (and shouldn't be) covered by PMS as it's private state data. Also the documentation hasn't been fully updated wrt changes in 2.2, and set handling is still not finalized. Finally as the issue of adding sets to the worldfile has already been raised in the past (in a different context though), as a result http://forums.gentoo.org/viewtopic-t-664181.html was created to gather community input about the preferred behavior, you may want to participate there.
In 2.2_pre6 package sets are now recorded in a separate physical file (which is still included in the "world" set though for portage).
