@Nonnull
  @Override
  public Sketch getResult() {
    return unionAll();
  }
@Test
  public void testEmptyAccumulator() {
    ThetaSketchAccumulator accumulator = new ThetaSketchAccumulator(_setOperationBuilder, 2);
    Assert.assertTrue(accumulator.isEmpty());
    Assert.assertEquals(accumulator.getResult().getEstimate(), 0.0);
  }