* 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/libexpr/expr-to-xml.hh

@@ -9,7 +9,7 @@
 
 namespace nix {
 
-void printTermAsXML(Expr e, std::ostream & out, ATermList & context);
+void printTermAsXML(Expr e, std::ostream & out, PathSet & context);
     
 }