files and hard-links them to each other to save disk space.

  Currently it doesn't actually do the hard-linking, it just reports
  the amount of space saved if it did.

  paths are missing, etc.

  distribution insofar they are applicable.

  http://www.daemonology.net/bsdiff/bsdiff-4.2.tar.gz) into the source
  tree.  The license is a bit peculiar, but it does allow verbatim
  copying, which is what we do here (i.e., so don't make any changes
  to the sources).

  manifests.

* Use the new patch downloader.

* Accept the NarHash line.
* Clear substitutes in `nix-channel --update'.

  registered substitute mappings.

  - Drop the store expression.  So now a substitute is just a
    command-line invocation (a program name + arguments).  If you
    register a substitute you are responsible for registering the
    expression that built it (if any) as a root of the garbage
    collector.
  - Drop the substitutes-rev DB table.

* Add /nix/var/nix/manifests directory.

  cheapest download path), as well as the hash of the contents of the
  path (necessary for checking patch applicability).

  substitute mechanism) creates a store path by downloading full NAR
  archives and/or patches specified in the available manifests.

  Any combination of present paths, full downloads, and patches can be
  used to construct the target path.  In particular, patches can be
  chained in sequence; and full NAR archives of the target path can be
  omitted (i.e., patch-only deployment is possible).  A shortest path
  algorithm is used to find the smallest set of files to be downloaded
  (the edge weights are currently file sizes, but one can imagine
  taking the network speed to the various source into account).

  Patches are binary deltas between two store paths.  To be precise,
  they are the output of the `bsdiff' program applied to the NAR
  archives obtained by dumping (`nix-store --dump') the two store
  paths.  The advantage of diff'ing NAR archives (and not, say, doing
  file-by-file diffs) is that file renames/moves are handled
  automatically.  The disadvantage is that we cannot optimise creation
  of unchanged files (by hard-linking).

  fetches.

  symlink.

  0, set the mode to either 444 or 555, set the group to the default).