unique.
* Drop `hashAlgo' attribute in manifests; prefix hashes with the hash
  algorithm instead.

  file names in the directory not included in any of the manifests
  specified on the command line.

* nix-prefecth-url: print out in base-16.

  expressions.

  conditional expression in the blacklist can specify when to
  continue/stop a traversal.  For example, in

    <condition>
      <within>
        <traverse>
          <not><hasAttr name='outputHash' value='.+' /></not>
        </traverse>
        <hasAttr name='outputHash' value='ef1cb003448b4a53517b8f25adb12452' />
      </within>
    </condition>

  we traverse the dependency graph, not following the dependencies of
  `fetchurl' derivations (as indicated by the presence of an
  `outputHash' attribute - this is a bit ugly).  The resulting set of
  paths is scanned for a fetch of a file with the given hash, in this
  case, the hash of zlib-1.2.1.tar.gz (which has a security bug).  The
  intent is that a dependency on zlib is not a problem if it is in a
  `fetchurl' derivation, since that's build-time only.  (Other
  build-time uses of zlib *might* be a problem, e.g., static linking.)

  Maybe this is a bad idea.

  checked against every item in a blacklist.

* Maintain the cleanup invariant in clearSubstitutes().

  store path (which is stored in the database).

  occurances of a component.  If the shortest path distance between a
  component P and Q in the referers graph is D, then the contribution
  of Q to the use of P is 1 / R^D, where R >= 1, typically 2.  This
  expresses that distant indirect uses are less important than nearby
  uses.

  For instance, this can disambiguate between the bootstrap GCC in
  Nixpkgs and the GCC of the final stdenv (the former has more uses,
  but they are further away),  and between the GCC of the final stdenv
  and the GCC+G77 build (the latter has very few uses).

  name but differ to much in sice (by more than a factor of 3), then
  never generate a patch.

  by the build farm.  See e.g.,
  http://catamaran.labs.cs.uu.nl/dist/nixpkgs-0.8/nixpkgs-0.7pre2302/;
  the user can click on packages, and they will be installed (assuming
  the `application/nix-package' MIME type has been associated with
  `nix-install-package').

  Nix expressions are no longer involved: a "package" is just a
  pointer to a manifest, and the top-level store derivation to be
  added to the user environment.  This makes these packages
  independent from Nix expression evolution.

  Note that we install the store derivation ($drvPath), not the
  resulting output path ($outPath).  This is equivalent, except that
  installing the derivation maintains the back-link from the output
  path to the derivation that built it.  This is useful for
  maintenance.

* Automatically re-exec in an xterm so that the user sees something
  when `nix-install-package' is run from a browser.

  too.
* Change the default hash for nix-prefetch-url back to md5, since
  that's what we use in Nixpkgs (for now; a birthday attack is rather
  unlikely there).  

  continue building when one fails unless `--keep-going' is
  specified.
* When `--keep-going' is specified, print out the set of failing
  derivations at the end (otherwise it can be hard to find out which
  failed).

  NAR dump of the path).

  in `fetchurl' in Nix <= 0.7, but doesn't in Nix 0.8.

  multiple times is also a top-level goal, then the second and later
  instantiations would never be created because there would be a
  stable pointer to the first one that would keep it alive in the
  WeakGoalMap.
* Some tracing code for debugging this kind of problem.