SetReducer implements Reducer<Set<T>> { @Override public Set<T> zero() { return ImmutableSet.of(); }  @Override Set<T> reduce(Set<T> v1, Set<T> v2); @Override Set<T> zero(); @Override boolean isZero(Set<T> ts);  }
@Test public void zeroShouldReturnEmptySet() { assertThat(reducer.zero()).isEmpty(); }