docs: public LongHashSet difference(final LongHashSet collection) {
          checkNotNull(collection);
  
          LongHashSet difference = null;
  
          final LongIterator it = iterator();
  
          while (it.hasNext()) {
              final long value = it.nextValue();
              if (!collection.contains(value)) {
                  if (difference == null) {
                      difference = new LongHashSet(size, missingValue);
                  }
  
                  difference.add(value);
              }
          }
  
          return difference;
      }