* Add some rejections and lexical restrictions to the grammar to make
  this work. 

  turned out to be a huge performance bottleneck (the text to printed
  would always be evaluated, even when it was above the verbosity
  level).  This reduces fix-ng execution time by over 50%.

  gprof(1) is very useful. :-)

* Logical operators (!, &&, ||, ->).

  substitutions, Fix terms are very large when printed as trees (in 
  memory, they are quite compact due to sharing).

  The ATerm library doesn't search the heap for pointers to ATerms
  when garbage collecting.  As a result, C++ containers such as
  `map<ATerm, ATerm>' will cause pointer to be hidden from the garbage
  collector, causing crashes.  Instead, we now use ATermTables.

  semicolons instead of comma's.  Final semicolon in the set is optional.

  attribute set.

* Mutually recursive attribute sets.
* Print evaluator efficiency statistics.

  buggy.  It fails to clear an internal variable (SG_textIndex)
  between invocations, so it can be called only once during a program
  execution.

  removed, paths are absolutised relative to the path containing the
  expression we just parsed, and integer literals are converted to
  actual integers.

  friends to do the parsing.  The parse table is embedded in the Fix
  executable using bin2c, which converts an arbitrary file into a C
  character array.

  writes stdout/stderr of the builder to ${prefix}/var/log/nix/x,
  where x is the file name of the derivation expression, e.g.,

    /nix/var/log/nix/54256391624be04fcb426048ae3ea0a4-d-pan-0.14.2.nix

  Note that consecutive builds of the same expression overwrite,
  rather than append to, existing log files.