anchor.diff:    src = fetchurl {
       url = "mirror://sourceforge/libpng/libpng16/${version}/${name}.tar.xz";
       hashOutput = false;
  -    sha256 = "232a602de04916b2b5ce6f901829caf419519e6a16cc9cd7c1c91187d3ee8b41";
  +    sha256 = "c918c3113de74a692f0a1526ce881dc26067763eb3915c57ef3a0f7b6886f59b";
     };
   
     buildInputs = [
  
positive.code: }:
  
  let
    version = "1.6.31";
  in
  stdenv.mkDerivation rec {
    name = "libpng-${version}";
  
negative.code:   patches = [
      (fetchurl {
        url = "mirror://sourceforge/libpng-apng/libpng16/${version}/${name}-apng.patch.gz";
        sha256 = "a6b79f1027bde510b215042c4375a3e2f9103113393269a5c261048e8d7a30c9";
      })
    ];
  
  