* Big cleanup of the semantics of paths, strings, contexts, string

concatenation and string coercion.  This was a big mess (see
  e.g. NIX-67).  Contexts are now folded into strings, so that they
  don't cause evaluation errors when they're not expected.  The
  semantics of paths has been clarified (see nixexpr-ast.def).
  toString() and coerceToString() have been merged.

  Semantic change: paths are now copied to the store when they're in a
  concatenation (and in most other situations - that's the
  formalisation of the meaning of a path).  So

    "foo " + ./bla

  evaluates to "foo /nix/store/hash...-bla", not "foo
  /path/to/current-dir/bla".  This prevents accidental impurities, and
  is more consistent with the treatment of derivation outputs, e.g.,
  `"foo " + bla' where `bla' is a derivation.  (Here `bla' would be
  replaced by the output path of `bla'.)

src/nix-instantiate/main.cc

@@ -41,7 +41,7 @@ static bool indirectRoot = false;
 static void printResult(EvalState & state, Expr e,
     bool evalOnly, bool xmlOutput, const ATermMap & autoArgs)
 {
-    ATermList context;
+    PathSet context;
     
     if (evalOnly)
         if (xmlOutput)