anchor.diff:  , libxcbxfixesExtlib ? true # X11 grabbing mouse rendering
   , libxcbshapeExtlib ? true # X11 grabbing shape rendering
   , lzma ? null # xz-utils
  -#, nvenc ? null # NVIDIA NVENC support
  +, nvenc ? false # NVIDIA NVENC support
  +  , nvidia-video-codec-sdk ? null
   , openal ? null # OpenAL 1.1 capture support
   #, opencl ? null # OpenCL code
   #, opencore-amr ? null # AMR-NB de/encoder & AMR-WB decoder
  
positive.code:     (enableFeature libxcbxfixesExtlib "libxcb-xfixes")
      (enableFeature libxcbshapeExtlib "libxcb-shape")
      (enableFeature (lzma != null) "lzma")
      #(enableFeature nvenc "nvenc")
      (enableFeature (openal != null) "openal")
      #(enableFeature opencl "opencl")
      #(enableFeature (opencore-amr != null && version3Licensing) "libopencore-amrnb")
  
negative.code: assert libxcbshmExtlib -> xorg.libxcb != null;
  assert libxcbxfixesExtlib -> xorg.libxcb != null;
  assert libxcbshapeExtlib -> xorg.libxcb != null;
  assert nvenc ->
    nvidia-video-codec-sdk != null
    && nonfreeLicensing;
  assert openglExtlib -> mesa != null;
  assert opensslExtlib -> gnutls == null && openssl != null && nonfreeLicensing;
  assert x11grabExtlib -> xorg.libX11 != null && xorg.libXv != null;
  