anchor.diff:  , gemfile ? null
   , lockfile ? null
   , gemset ? null
  +, allBins ? false
   , ruby ? defs.ruby
   , gemConfig ? defaultGemConfig
   , postBuild ? null
  
positive.code:     paths = envPaths;
      pathsToLink = [ "/lib" ];
  
      postBuild = ''
        ${ruby}/bin/ruby ${./gen-bin-stubs.rb} \
          "${ruby}/bin/ruby" \
          "${confFiles}/Gemfile" \
          "$out/${ruby.gemPath}" \
          "${bundler}/${ruby.gemPath}" \
          ${lib.escapeShellArg binPaths} \
          ${lib.escapeShellArg groups}
      '' + lib.optionalString (postBuild != null) postBuild;
  
      meta = { platforms = ruby.meta.platforms; } // meta;
  
  
negative.code:           require 'bundler/setup'
          '';
          in stdenv.mkDerivation {
            name = "${drvName}-interactive-environment";
            nativeBuildInputs = [ wrappedRuby bundlerEnv ];
            shellHook = ''
              export OLD_IRBRC="$IRBRC"
  