}
    // Should overflow without throwing.
    accumulator.addAll(accumulator.snapshot());
    assertThat(accumulator.count()).isLessThan(0L);
  }

  public void testMean() {
    assertThrows(IllegalStateException.class, () -> emptyAccumulator.mean());
    assertThrows(IllegalStateException.class, () -> emptyAccumulatorByAddAllEmptyIterable.mean());