Perhaps a new concept of a "symlink with backups" could be introduced. You install ProgA and the lib is symlinked into the index You install ProgB and the inde

Perhaps a new concept of a "symlink with backups" could be introduced. You install ProgA and the lib is symlinked into the index You install ProgB and the index's symlink to ProgA's lib gets some information added to it along the lines of "if the symlink to ProgA/lib is broken, try ProgB/lib" Then when you uninstall ProbA the fallback symlink is activated. At some point you would want to clean up the dangling primary link but maybe the best time to do it is immedately. i.e. the first time the primary fails you make the backup the new primary and now there's no backup, i.e. it looks the way it would look if you had only ever installed ProgB.