* Sets.cartesianProduct with those, then transforms the result to unwrap the Optionals.
   */
  private Iterable<List<Object>> buildCartesianProduct(Set<?>... sets) {
    List<Set<Optional<?>>> optionalSets = Lists.newArrayListWithExpectedSize(sets.length);
    for (Set<?> set : sets) {
      Set<Optional<?>> optionalSet =
          Sets.newLinkedHashSet(Iterables.transform(set, Optional::fromNullable));
      optionalSets.add(optionalSet);
    }

        Thread.yield();
      }
    }
    gettersStartedSignal.countDown();
    gettersComplete.await();

    List<Object> resultList = Lists.newArrayListWithExpectedSize(nThreads);
    for (int i = 0; i < nThreads; i++) {
      resultList.add(result.get(i));
    }
    return resultList;
  }

        Thread.yield();
      }
    }
    gettersStartedSignal.countDown();
    gettersComplete.await();

    List<Object> resultList = Lists.newArrayListWithExpectedSize(nThreads);
    for (int i = 0; i < nThreads; i++) {
      resultList.add(result.get(i));
    }
    return resultList;
  }