* New language feature: with expressions.

The expression `with E1; E2' evaluates to E2 with all bindings in
  the attribute set E1 substituted.  E.g.,

    with {x = 123;}; x

  evaluates to 123.  That is, the attribute set E1 is in scope in E2.

  This is particularly useful when importing files containing lots
  definitions.  E.g., instead of

    let {
      inherit (import ./foo.nix) a b c d e f;

      body = ... a ... f ...;
    }

  we can now say

    with import ./foo.nix;

    ... a ... f ...

  I.e., we don't have to say what variables should be brought into scope.

src/libexpr/primops.cc

@@ -205,7 +205,7 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
         try {
             processBinding(state, value, ne, ss);
         } catch (Error & e) {
-            throw Error(format("while processing derivation attribute `%1%' at %2%:\n%3%")
+            throw Error(format("while processing the derivation attribute `%1%' at %2%:\n%3%")
                 % key % showPos(pos) % e.msg());
         }