anchor.diff:  
     useFromStdenv = it : alternative : if (builtins.hasAttr it stdenv) then
       (builtins.getAttr it stdenv) else alternative;
  -
  -  lib = import ../lib;
  -
  -  # Return an attribute from the Nixpkgs configuration file, or
  -  # a default value if the attribute doesn't exist.
  -  getConfig = attrPath: default: lib.getAttr attrPath default config;
   
     # Return the first available value in the order: pkg.val, val, or default.
     getPkgConfig = pkg : val : default : (getConfig [ pkg val ] (getConfig [ val ] default));
  
positive.code: 
    myEnvFun = import ../misc/my_env;
  
    myPackages = recurseIntoAttrs ( ( getConfig ["myPackages"] (x : {}) ) pkgs );
  
  }; in pkgs
  
negative.code:   ### Helper functions.
  
  
    inherit lib config getConfig;
    
    # Override the compiler in stdenv for specific packages.
    overrideGCC = stdenv: gcc: stdenv //
      { mkDerivation = args: stdenv.mkDerivation (args // { NIX_GCC = gcc; });
  