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