Conceptually all the installed packages form a repo of installed packages. But no repo of installed packages is supported. There is installed packages db but it's not the same as the repo of installed ebuilds. The suggestion is to extend installed packages db in a way it would keep all the patches and distfiles (distfiles - optional) of the installed ebuilds and emerge would consider that db as a repo name it "installed". That would give an opportunity to re-configure gentoo to your liking even if the portage tree has no longer support of the currently installed applications and the ebuilds and patches and distfiles are removed. IMHO the current emerge is walking around that concept Reproducible: Always
*** This bug has been marked as a duplicate of bug 703058 ***