This was intended to provide a mechanism to remove old files from an enduser's cache, where version number updates result in files which will never be seen.
find aggregations within the user's history. This currently looks for $Y, but aggregations can also be $y, etc.
Return an array of files where newer versions prevent the older from being used. This will not look for version constraints (e.g. $(v,lt=2)), so use with some care.