anchor.diff:  
   let
     # bump whenever fontconfig breaks compatibility with older configurations
  -  configVersion = "2.12";
  +  configVersion = "2.13";
   in
   stdenv.mkDerivation rec {
  -  name = "fontconfig-2.12.6";
  +  name = "fontconfig-2.13.0";
   
     src = fetchurl {
       urls = [
         "https://www.freedesktop.org/software/fontconfig/release/${name}.tar.bz2"
         "http://fontconfig.org/release/${name}.tar.bz2"
       ];
  -    sha256 = "cf0c30807d08f6a28ab46c61b8dbd55c97d2f292cf88f3a07d3384687f31f017";
  +    sha256 = "91dde8492155b7f34bb95079e79be92f1df353fcc682c19be90762fd3e12eeb9";
     };
   
     nativeBuildInputs = [
  
positive.code:     mv fonts.conf.tmp $out/etc/fonts/fonts.conf
    '';
  
    doCheck = true;
  
    passthru = {
      inherit configVersion;
  
negative.code: 
    configureFlags = [
      "--enable-iconv"
      "--disable-docs"
      # This is what you get when loading fontconfig's config fails
      # for any reason.
  