* Another experimental feature: a way to truly override attributes in

a rec.  This will be very useful to allow end-user customisation of
  all-packages.nix, for instance globally overriding GCC or some other
  dependency.  The // operator doesn't cut it: you could replace the
  "gcc" attribute, but all other attributes would continue to
  reference the original value due to the substitution semantics of
  rec.

  The syntax is a bit hacky but this is to allow backwards
  compatibility.

src/libexpr/nixexpr-ast.def

@@ -93,3 +93,4 @@ AttrRHS | Expr Pos | ATerm |
 
 eTrue = makeBool(makeTrue())
 eFalse = makeBool(makeFalse())
+sOverrides = toATerm("__overrides")