public List<?> get() {
      return new LinkedList<>();
    }
  }

  private static final class HashSetSupplier<V extends @Nullable Object>
      implements Supplier<Set<V>>, Serializable {
    private final int expectedValuesPerKey;

    HashSetSupplier(int expectedValuesPerKey) {
      this.expectedValuesPerKey = checkNonnegative(expectedValuesPerKey, "expectedValuesPerKey");
    }