public static MultimapBuilderWithKeys<@Nullable Object> hashKeys() {
    return hashKeys(DEFAULT_EXPECTED_KEYS);
  }

  /**
   * Uses a hash table to map keys to value collections, initialized to expect the specified number
   * of keys.
   *
   * @throws IllegalArgumentException if {@code expectedKeys < 0}
   */
  public static MultimapBuilderWithKeys<@Nullable Object> hashKeys(int expectedKeys) {

  // but types are typically only Comparable to themselves).
  @SuppressWarnings({"rawtypes", "unchecked"})
  private static MultimapBuilderWithKeys<Comparable<?>> rawtypeToWildcard(
      MultimapBuilderWithKeys<Comparable> treeKeys) {
    return (MultimapBuilderWithKeys) treeKeys;
  }

  @SuppressWarnings({"rawtypes", "unchecked"})
  private static <K extends @Nullable Object>