@Nonnull
  @Override
  public Sketch getResult() {
    return unionAll();
  }
@Test
  public void testUnionWithEmptyInput() {
    ThetaSketchAccumulator accumulator = new ThetaSketchAccumulator(_setOperationBuilder, 3);
    ThetaSketchAccumulator emptyAccumulator = new ThetaSketchAccumulator(_setOperationBuilder, 3);

    accumulator.merge(emptyAccumulator);

    Assert.assertTrue(accumulator.isEmpty());
    Assert.assertEquals(accumulator.getResult().getEstimate(), 0.0);
  }