@Override
  public Coder<VarianceAccumulator> getAccumulatorCoder(
      CoderRegistry registry, Coder<T> inputCoder) {
    return SerializableCoder.of(VarianceAccumulator.class);
  }
@Test
  public void testCreatesAccumulatorCoder() {
    assertNotNull(varianceFn.getAccumulatorCoder(CoderRegistry.createDefault(), VarIntCoder.of()));
  }