anchor.diff:  
   let version = "1.2.8"; in
   
  -stdenv.mkDerivation (rec {
  +stdenv.mkDerivation rec {
     name = "zlib-${version}";
   
     src = fetchurl {
  
positive.code:     license = licenses.zlib;
      platforms = platforms.all;
    };
  } // (if stdenv.isDarwin then {
    postInstall = ''
      # jww (2015-01-06): Sometimes this library install as a .so, even on
      # Darwin; others time it installs as a .dylib.  I haven't yet figured out
      # what causes this difference.
      for file in $out/lib/*.so* $out/lib/*.dylib* ; do
        install_name_tool -id "$file" $file
      done
    '';
  } else {}))
  
negative.code:       ];
      sha256 = "039agw5rqvqny92cpkrfn243x2gd4xn13hs3xi6isk55d2vqqr9n";
    };
  
    configureFlags = [
      (if static then "--static" else "")
  