anchor.diff:  
   let
     inherit (pkgs) stdenv;
  -  inherit (pkgs.lib) showVal;
  +  inherit (pkgs.lib) showVal optional;
     # This expression builds the raw C headers and source files for the base
     # node.js installation. Node packages which use the C API for node need to
     # link against these files and use the headers.
  
positive.code: 
        buildInputs = [npm] ++ buildInputs ++
                      attrValues _devDependencies ++
                      neededNatives;
      };
  
      in stdenv.mkDerivation mkDerivationArgs;
  
negative.code:         # NPM reads the `HOME` environment variable and fails if it doesn't
          # exist, so set it here.
          export HOME=$PWD
          echo npm install ${npmFlags}
  
          # Try doing the install first. If it fails, first check the
          # dependencies, and if we don't uncover anything there just rerun it
          # with verbose output.
          npm install ${npmFlags} >/dev/null 2>&1 || {
            echo "Installation of ${name}@${version} failed!"
            echo "Checking dependencies to see if any aren't satisfied..."
            node ${./checkPackageJson.js} checkDependencies
            echo "Dependencies seem ok. Rerunning with verbose logging:"
            npm install . ${npmFlags} --loglevel=verbose
            if [[ -d node_modules ]]; then
              echo "node_modules contains these files:"
              ls -l node_modules
  