anchor.diff:    src = fetchurl {
       url = "mirror://sourceforge/libpng/libpng16/${version}/${name}.tar.xz";
       hashOutput = false;
  -    sha256 = "2f1e960d92ce3b3abd03d06dfec9637dfbd22febf107a536b44f7a47c60659f6";
  +    sha256 = "23912ec8c9584917ed9b09c5023465d71709dce089be503c7867fec68a93bcd7";
     };
   
     buildInputs = [
  
positive.code: }:
  
  let
    version = "1.6.34";
  in
  stdenv.mkDerivation rec {
    name = "libpng-${version}";
  
negative.code:   patches = [
      (fetchurl {
        url = "mirror://sourceforge/libpng-apng/libpng16/${version}/${name}-apng.patch.gz";
        sha256 = "18ceb239857bbd32ca706cdad29f1a5dcfb87a5c54258db3bb077cff78c12e04";
      })
    ];
  
  