docs: public IntHashSet difference(final IntHashSet other)
      {
          IntHashSet difference = null;
  
          for (final int value : values)
          {
              if (value != MISSING_VALUE && !other.contains(value))
              {
                  if (difference == null)
                  {
                      difference = new IntHashSet();
                  }
  
                  difference.add(value);
              }
          }
  
          if (other.containsMissingValue && !this.containsMissingValue)
          {
              if (difference == null)
              {
                  difference = new IntHashSet();
              }
  
              difference.add(MISSING_VALUE);
          }
  
          return difference;
      }