anchor.diff:  { stdenv, fetchurl, makeDesktopItem
   , jre, libX11, libXext, libXcursor, libXrandr, libXxf86vm
  +, openjdk
   , mesa, openal
   , useAlsa ? false, alsaOss ? null }:
   with stdenv.lib;
  
positive.code: 
      mkdir -p $out/share/applications
      ln -s ${desktopItem}/share/applications/* $out/share/applications/
    '';
  
    meta = {
  
negative.code: assert useAlsa -> alsaOss != null;
  
  let
    desktopItem = makeDesktopItem {
      name = "minecraft";
      exec = "minecraft";
      icon = "minecraft";
      comment = "A sandbox-building game";
      desktopName = "Minecraft";
      genericName = "minecraft";
  