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