* Command `nix ensure' which is like `nix getpkg' except that if the
has refers to a run action it will just ensure that the imports are
there.
* Command `nix closure' to print out the closure of the set of
descriptors under the import relation, starting at a set of roots.
This can be used for garbage collection (e.g., given a list of
`activated' packages, we can delete all packages not reachable from
those).
* Command `nix graph' to print out a Dot graph of the dependency
graph.
* `nix-addroot' adds a root for the (unimplemented) garbage collector.
