protected @Nullable Pair<String, Fix> validate(List<? extends Item> current, List<? super String> warnings) {
    String error = null;
    for (int i = 0; i < current.size(); i++) {
      Item item = current.get(i);
      String s = validate(item);
      warnings.set(i, s);
      if (error == null) {
        error = s;
      }
    }
    return error != null ? Pair.create(error, (Fix)null) : null;
  }
protected @Nullable Pair<String, Fix> validate(List<? extends Item> current, List<? super String> warnings)