}
    };

    abstract <T extends @Nullable Object, A extends @Nullable Object, R extends @Nullable Object>
        A result(Collector<T, A, R> collector, Iterable<T> inputs);
  }

  /**
   * Verifies that the specified expected result is always produced by collecting the specified
   * inputs, regardless of how the elements are divided.
   */
  @SafeVarargs
  @CanIgnoreReturnValue