anchor.diff:      the store using e.g.:
   
         releaseType=alpha
  -      version=0.16.51
  +      version=0.17.74
         nix-prefetch-url file://$HOME/Downloads/factorio_\''${releaseType}_x64_\''${version}.tar.xz --name factorio_\''${releaseType}_x64-\''${version}.tar.xz
   
       Note the ultimate "_" is replaced with "-" in the --name arg!
  
positive.code: { stdenv, fetchurl, makeWrapper
  , alsaLib, libX11, libXcursor, libXinerama, libXrandr, libXi, libGL
  , factorio-utils
  , releaseType
  , mods ? []
  
negative.code:     headless = base;
      demo = base // {
  
        buildInputs = [ makeWrapper libpulseaudio ];
  
        libPath = stdenv.lib.makeLibraryPath [
          alsaLib
          libpulseaudio
          libX11
          libXcursor
          libXinerama
  