anchor.diff:  assert zlib != null;
   
   let
  -  version = "1.6.34";
     patchVersion = "1.6.34";
  -  sha256 = "1xjr0v34fyjgnhvaa1zixcpx5yvxcg4zwvfh0fyklfyfj86rc7ig";
     patch_src = fetchurl {
       url = "mirror://sourceforge/libpng-apng/libpng-${patchVersion}-apng.patch.gz";
       sha256 = "1ha4npf9mfrzp0srg8a5amks5ww84xzfpjbsj8k3yjjpai798qg6";
  
positive.code: 
  in stdenv.mkDerivation rec {
    name = "libpng" + whenPatched "-apng" + "-${version}";
  
    src = fetchurl {
      url = "mirror://sourceforge/libpng/libpng-${version}.tar.xz";
      inherit sha256;
    };
    postPatch = whenPatched "gunzip < ${patch_src} | patch -Np1";
  
  
negative.code: 
    propagatedBuildInputs = [ zlib ];
  
    doCheck = stdenv.hostPlatform == stdenv.buildPlatform;
  
    passthru = { inherit zlib; };
  
  