anchor.diff:  
   common =
     { lib, stdenv, fetchpatch, perl, curl, bzip2, sqlite, openssl ? null, xz
  -  , pkgconfig, boehmgc, perlPackages, libsodium, brotli, boost, editline
  +  , pkgconfig, boehmgc, perlPackages, libsodium, brotli, boost, editline, nlohmann_json
     , autoreconfHook, autoconf-archive, bison, flex, libxml2, libxslt, docbook5, docbook_xsl_ns, jq
     , busybox-sandbox-shell
     , storeDir
  
positive.code: 
    nixFlakes = lib.lowPrio (callPackage common rec {
      name = "nix-2.4${suffix}";
      suffix = "pre20190922_382aa05";
      src = fetchFromGitHub {
        owner = "NixOS";
        repo = "nix";
        rev = "382aa05ff71b61379f5c2792eaf517bdf4a5c5bf";
        hash = "sha256-k4vV3Q1YVmLd+49AETnsSGetpDjD6sdd9yBrnpi8Q3g=";
      };
      fromGit = true;
  
  
negative.code:         ++ lib.optionals (!is20) [ curl perl ]
          ++ lib.optionals fromGit [ autoreconfHook autoconf-archive bison flex libxml2 libxslt docbook5 docbook_xsl_ns jq ];
  
        buildInputs = [ curl openssl sqlite xz bzip2 nlohmann_json ]
          ++ lib.optional (stdenv.isLinux || stdenv.isDarwin) libsodium
          ++ lib.optionals is20 [ brotli boost editline ]
          ++ lib.optional withLibseccomp libseccomp
  