futures.add(pool.submit(new MutateTask(multiset, keys)));
      }

      int[] deltas = new int[3];
      for (Future<int[]> future : futures) {
        int[] taskDeltas = future.get();
        for (int i = 0; i < deltas.length; i++) {
          deltas[i] += taskDeltas[i];
        }
      }

      List<Integer> actualCounts = transform(keys, multiset::count);