class Env {

  public void requireAllVarsUsed() {
    if (!unused.isEmpty()) {
      throw new IllegalArgumentException("Env vars set but not used: "
          // add prefix back in to get actual env vars
          + unused.stream().map(key -> prefix + key).collect(Collectors.toList()).toString());
    }
  }

  public  Env(Set<String> include);
  public  Env(Set<String> include, String prefix);

}