+ index, the space saving is less compelling.
+
+ - A user requested what's essentially a VFS layer: given multiple
+ index files and a map of how they fit into an overall namespace,
+ we should be able to construct the right answers for any query
+ about the resulting aggregated hierarchy by doing at most
+ O(number of indexes * normal number of queries) work.
+
+ - Support for filtering the scan by ownership and permissions. The
+ index data structure can't handle this, so we can't build a
+ single index file admitting multiple subset views; but a user
+ suggested that the scan phase could record information about
+ ownership and permissions in the dump file, and then the indexing
+ phase could filter down to a particular sub-view - which would at
+ least allow the construction of various subset indices from one
+ dump file, without having to redo the full disk scan which is the
+ most time-consuming part of all.