code: @CanIgnoreReturnValue
    @Override
    public int setCount(@Nullable E element, int count) {
      checkNonnegative(count, "count");
  
      Count existingCounter;
      int oldCount;
      if (count == 0) {
        existingCounter = backingMap.remove(element);
        oldCount = getAndSet(existingCounter, count);
      } else {
        existingCounter = backingMap.get(element);
        oldCount = getAndSet(existingCounter, count);
  
        if (existingCounter == null) {
          backingMap.put(element, new Count(count));
        }
      }
  
      size += (count - oldCount);
      return oldCount;
    }
docs: Roughly a 33% performance improvement over AbstractMultiset.setCount().