anchor.diff:  
   with stdenv.lib;
   
  -stdenv.mkDerivation {
  +stdenv.mkDerivation rec {
     name = "bup-${version}";
   
     src = fetchzip {
  
positive.code: { stdenv, fetchzip, python, pyxattr, pylibacl, setuptools, fuse, git, perl, pandoc, makeWrapper
  , par2cmdline, par2Support ? false }:
  
  assert par2Support -> par2cmdline != null;
  
  
negative.code:   buildInputs = [ python git ];
    nativeBuildInputs = [ pandoc perl makeWrapper ];
  
    darwin_10_10_patch = fetchurl {
      url = "https://github.com/bup/bup/commit/75d089e7cdb7a7eb4d69c352f56dad5ad3aa1f97.diff";
      sha256 = "05kp47p30a45ip0fg090vijvzc7ijr0alc3y8kjl6bvv3gliails";
    };
  
    postPatch = ''
      patchShebangs .
      substituteInPlace Makefile --replace "-Werror" ""
      substituteInPlace Makefile --replace "./format-subst.pl" "perl ./format-subst.pl"
    '' + optionalString par2Support ''
      substituteInPlace cmd/fsck-cmd.py --replace "['par2'" "['${par2cmdline}/bin/par2'"
    '' + optionalString (elem stdenv.system platforms.darwin) ''
      patch -p1 < ${darwin_10_10_patch}
    '';
  
    dontAddPrefix = true;
  