anchor.diff:  { stdenv, fetchurl, unzip, patchelf, xorg, openal }:
   
   let
  +  urls = file:
  +    [
  +      # Untrusted mirrors - do not update hashes
  +      "https://ludios.org/mirror/ue4demos/${file}"
  +      "http://web.archive.org/web/20140824192039/http://ue4linux.raxxy.com/${file}"
  +    ];
  +
     buildDemo = { name, src }:
       stdenv.mkDerivation rec {
         inherit name src;
  
positive.code:   scifi_hallway_demo = buildDemo rec {
      name = "ue4demos-scifi_hallway_demo";
      src = fetchurl {
        url = "http://ue4linux.raxxy.com/sci-fi_hallway_demo.zip";
        sha256 = "14qp9iwm47awn8d9j6ijh6cnds308x60xs4vi2fvz2666jlz1pq2";
      };
    };
  
negative.code:   realistic_rendering = buildDemo rec {
      name = "ue4demos-realistic_rendering";
      src = fetchurl {
        urls = urls "realistic_rendering_demo.zip";
        sha256 = "0r16nznkv475hkw5rnngqsc69ch8vh86dppyyyr9nn43dkr2110a";
      };
    };
  