anchor.diff:  else
   
   let
  -  version = "9.0.3";
  +  version = "9.1";
   in
   stdenv.mkDerivation {
     name = "mesa-${version}";
   
     src = fetchurl {
       url = "ftp://ftp.freedesktop.org/pub/mesa/${version}/MesaLib-${version}.tar.bz2";
  -    sha256="1986av3pl8v9hhxxqqdm2wn3qlwwnrznvfmm4wszhyf3n82h157a";
  +    sha256="0yvhl0vdg32h0xr4xi348gkp0hlcc16j1cfxn4pyc9pywyzlqj5g";
     };
   
     prePatch = "patchShebangs .";
  
positive.code:     + stdenv.lib.optionalString enableTextureFloats " --enable-texture-float";
  
    buildInputs = [
      autoconf automake libtool expat libxml2Python udev llvm
      libdrm libXxf86vm libXfixes libXdamage glproto dri2proto libX11 libXext libxcb libXt
    ];
  
  
negative.code: { stdenv, fetchurl, flex, bison, pkgconfig, intltool, libdrm, file, expat, makedepend
  , libXxf86vm, libXfixes, libXdamage, glproto, dri2proto, libX11, libxcb, libXext
  , libXt, udev, enableTextureFloats ? false, enableR600LlvmCompiler ? false
  , python, libxml2Python, autoconf, automake, libtool, llvm, writeText }:
  