anchor.diff:  
   with stdenv.lib;
   
  -stdenv.mkDerivation {
  +stdenv.mkDerivation rec {
     name = "bup-${version}";
   
     src = fetchzip {
  
positive.code:   buildInputs = [ python git ];
    nativeBuildInputs = [ pandoc perl makeWrapper ];
  
    patchPhase = ''
      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'"
    '';
  
    dontAddPrefix = true;
  
negative.code: { stdenv, fetchzip, fetchurl, python, pyxattr, pylibacl, setuptools
  , fuse, git, perl, pandoc, makeWrapper, par2cmdline, par2Support ? false }:
  
  assert par2Support -> par2cmdline != null;
  
  