anchor.diff:    meta = {
       description = "A C++ cross-platform lightweight GUI library";
       homepage = http://www.fltk.org;
  -    platforms = stdenv.lib.platforms.linux;
  +    platforms = stdenv.lib.platforms.linux ++ stdenv.lib.platforms.darwin;
       license = stdenv.lib.licenses.gpl2;
     };
   
   }
  -
  
positive.code: { stdenv, composableDerivation, fetchurl, pkgconfig, xlibsWrapper, inputproto, libXi
  , freeglut, mesa, libjpeg, zlib, libXinerama, libXft, libpng
  , cfg ? {}
  }:
  
  let inherit (composableDerivation) edf; in
  
negative.code:   '';
  
    nativeBuildInputs = [ pkgconfig ];
    propagatedBuildInputs = [ inputproto ]
      ++ (if stdenv.isDarwin
          then (with darwin.apple_sdk.frameworks; [Cocoa AGL GLUT freetype libtiff])
          else [ xlibsWrapper libXi freeglut ]);
  
    enableParallelBuilding = true;
  
  