* `make dist'.

  substituting for (obvious, really).

* For greater efficiency, nix-pull/unnar will place the output in a
  path that is probably the same as what is actually needed, thus
  preventing a path copy.

* Even if a output id is given in a Fix package expression, ensure
  that the resulting Nix derive expression has a different id.  This
  is because Nix expressions that are semantically equivalent (i.e.,
  build the same result) might be different w.r.t. efficiency or
  divergence.  It is absolutely vital for the substitute mechanism
  that such expressions are not used interchangeably.

  corresponding to the given id.

* When pushing, put the hash in the file name so that the 
  client can verify (proof-carrying file names?).

  (There should really be a switch for this).

  value; this potentially dangerous feature enables better 
  sharing for those paths for which the content is known in 
  advance (e.g., because a content hash is given).
* Fast builds: if we can expand all output paths of a derive 
  expression, we don't have to build.

* A function to find all Nix expressions whose output ids are
  completely contained in some set.  Useful for uploading relevant Nix
  expressions to a shared cache.

  read()/write() to fix this once and for all.

  database and store.

  number of bytes, e.g., in case of a signal like SIGSTOP.  
  This caused `nix --dump' to fail sometimes.

  Note that this bug went unnoticed because the call to `nix 
  --dump' is in a pipeline, and the shell ignores non-zero 
  exit codes from all but the last element in the pipeline.  
  Is there any way to check the result of the initial elements
  in the pipeline?  (In other words, is it at all possible to 
  write reliable shell scripts?)

  input path is referenced in an output paths, we also have to 
  add all ids referenced by that input path.
* Better debug assertions to catch these sorts of errors.

* Fix -qp query.

  This is not entirely trivial since this introduces the possibility
  of mutual recursion.
* Made normal forms self-contained.
* Use unique ids, not content hashes, for content referencing.

  opposed to declared references.  This prunes the reference
  graph, thus allowing better garbage collection and more
  efficient derivate distribution.

  through config.h, to prevent silly Autoconf problems.

  etc. correctly.
* Fixed nix-switch.

  returning different paths if the hash of the path to be added was
  already available in the store under a different name).

  number of bytes in one call).

  Unifying substitutes and successors isn't very feasible for now,
  since substitutes are only used when no path with a certain is
  known.  Therefore, a normal form of some expression stored as a
  substitute would not be used unless the expression itself was
  missing.