anchor.diff:  { lib, stdenv, fetchurl, fetchFromGitHub, cmake, pkgconfig, xorg, libGLU
   , libGL, glew, ocl-icd, python3
   , cudaSupport ? false, cudatoolkit
  +, darwin
   }:
   
   stdenv.mkDerivation rec {
  
positive.code:   meta = {
      description = "An Open-Source subdivision surface library";
      homepage = http://graphics.pixar.com/opensubdiv;
      platforms = lib.platforms.linux;
      maintainers = [ lib.maintainers.eelco ];
      license = lib.licenses.asl20;
    };
  
negative.code:   outputs = [ "out" "dev" ];
  
    buildInputs =
      [ cmake pkgconfig libGLU libGL python3
        # FIXME: these are not actually needed, but the configure script wants them.
        glew xorg.libX11 xorg.libXrandr xorg.libXxf86vm xorg.libXcursor
        xorg.libXinerama xorg.libXi
      ]
      ++ lib.optional (!stdenv.isDarwin) ocl-icd
      ++ lib.optionals stdenv.isDarwin (with darwin.apple_sdk.frameworks; [OpenCL Cocoa CoreVideo IOKit AppKit AGL ])
      ++ lib.optional cudaSupport cudatoolkit;
  
    cmakeFlags =
      [ "-DNO_TUTORIALS=1"
        "-DNO_REGRESSION=1"
        "-DNO_EXAMPLES=1"
        "-DNO_METAL=1" # don’t have metal in apple sdk
      ] ++ lib.optionals (!stdenv.isDarwin) [
        "-DGLEW_INCLUDE_DIR=${glew.dev}/include"
        "-DGLEW_LIBRARY=${glew.dev}/lib"
      ] ++ lib.optionals cudaSupport [
  