public MeanStatistic copy() {
    return new MeanStatistic(this);
  }
@Test
  public void testCopyEmpty() throws Throwable {
    MeanStatistic stat = empty.copy();
    Assertions.assertThat(stat)
        .describedAs("copy of " + empty)
        .isEqualTo(empty)
        .isNotSameAs(empty);
  }