@Nonnull
  @Override
  public Sketch getResult() {
    return unionAll();
  }
@Test
  public void testAccumulatorWithSingleSketch() {
    UpdateSketch input = Sketches.updateSketchBuilder().build();
    IntStream.range(0, 1000).forEach(input::update);
    Sketch sketch = input.compact();

    ThetaSketchAccumulator accumulator = new ThetaSketchAccumulator(_setOperationBuilder, 2);
    accumulator.apply(sketch);

    Assert.assertFalse(accumulator.isEmpty());
    Assert.assertEquals(accumulator.getResult().getEstimate(), sketch.getEstimate());
  }