code: func (s *OrderedSet) Difference(other *OrderedSet) *OrderedSet {
  	diff := NewOrderedSetWithCapacity(s.Cardinality())
  	for _, e := range s.s {
  		if !other.Contains(e) {
  			diff.Add(e)
  		}
  	}
  
  	return diff
  }
docs: // Difference returns the elements that are in this set, but not included in
  // other.