let pkgs = import <nixpkgs> {};
in

if pkgs == null then
builtins.fetchGit {
  url = "https://github.com/nixos/nixpkgs";
  ref = "master";
  rev = "da4c6be0187a694bdeb3efc28b29ee0e4c30702f";
  shallow = true;
}
else pkgs.fetchgit {
  url = "https://github.com/nixos/nixpkgs";
  rev = "da4c6be0187a694bdeb3efc28b29ee0e4c30702f";
  sha256 = "sha256-1rcG6x0vKnnzGhABPg/QvL75DzhJxM810wZKAukoF1M=";
}