anchor.diff:    useVersion = name: f: f {
       version = getConfig [ "environment" "versions" name ];
     };
  -
  -  # The contents of the configuration file found at $NIXPKGS_CONFIG or
  -  # $HOME/.nixpkgs/config.nix.
  -  config =
  -    let {
  -      toPath = builtins.toPath;
  -      getEnv = x: if builtins ? getEnv then builtins.getEnv x else "";
  -      pathExists = name:
  -        builtins ? pathExists && builtins.pathExists (toPath name);
  -
  -      configFile = getEnv "NIXPKGS_CONFIG";
  -      homeDir = getEnv "HOME";
  -      configFile2 = homeDir + "/.nixpkgs/config.nix";
  -
  -      body =
  -        if configFile != "" && pathExists configFile
  -        then import (toPath configFile)
  -        else if homeDir != "" && pathExists configFile2
  -        then import (toPath configFile2)
  -        else {};
  -    };
   
     # Change the symbolic name of a package for presentation purposes
     # (i.e., so that nix-env users can tell them apart).
  
positive.code:   ### Helper functions.
  
  
    # Override the compiler in stdenv for specific packages.
    overrideGCC = stdenv: gcc: stdenv //
      { mkDerivation = args: stdenv.mkDerivation (args // { NIX_GCC = gcc; });
  
negative.code: 
    myEnvFun = import ../misc/my_env;
  
  }; in pkgs
  