self: super: {
  util = rec {
    dirEntries = path: builtins.attrNames (builtins.readDir path);
    mapHosts = f: builtins.map f (dirEntries ../hosts);
    mapHostAttrs = key_f: val_f:
      builtins.listToAttrs (mapHosts (host: {
        name = key_f host;
        value = val_f host;
      }));
    mapObjKeys = obj: f: builtins.map f (builtins.attrNames obj);
    mapKeys = obj: f: builtins.map f obj;
    nixFilesIn = path:
      builtins.map (e: import (path + /${e}))
      (builtins.filter (x: (builtins.match ".*\\.nix$" x) != null)
        (dirEntries path));
    without = exclusions: list:
      builtins.filter (x: !builtins.elem x exclusions) list;
  };
}