code: func (set StringSet) Difference(other StringSet) StringSet {
  	differencedSet := NewStringSet()
  	for elem := range set {
  		if !other.Contains(elem) {
  			differencedSet.Add(elem)
  		}
  	}
  	return differencedSet
  }
docs: // Returns a new set with items in the current set but not in the other set