anchor.diff:  x@{builderDefsPackage
     , unzip, cmake, mesa, freeglut, libX11, xproto
  -  , inputproto, libXi
  +  , inputproto, libXi, fetchsvn, pkgconfig
     , ...}:
   builderDefsPackage
   (a :  
  
positive.code: 
    buildInputs = map (n: builtins.getAttr n x)
      (builtins.attrNames (builtins.removeAttrs x helperArgNames));
    sourceInfo = rec {
      baseName="box2d";
      version="2.1.2";
      name="${baseName}-${version}";
      url="http://box2d.googlecode.com/files/Box2D_v${version}.zip";
      hash="0m5szd74ig8yqazwk2g3zl4z7wwp08k52awawk1pigy6a4z1qd9v";
    };
  in
  rec {
    src = a.fetchurl {
      url = sourceInfo.url;
      sha256 = sourceInfo.hash;
    };
  
    inherit (sourceInfo) name version;
    inherit buildInputs;
  
    phaseNames = ["changeSettings" "doCmake" "doMakeInstall"];
  
negative.code:     platforms = with a.lib.platforms;
        linux;
      license = "bsd";
      inherit version;
    };
  }) x
  
  