docs: public IntHashSet difference(final IntHashSet collection) {
          checkNotNull(collection, "Collection must not be null");
  
          IntHashSet difference = null;
  
          final IntIterator it = iterator();
  
          while (it.hasNext()) {
              final int value = it.nextValue();
              if (!collection.contains(value)) {
                  if (difference == null) {
                      difference = new IntHashSet(size, missingValue);
                  }
  
                  difference.add(value);
              }
          }
  
          return difference;
      }