code: func (s *Set) Difference(other *Set) *Set {
  	result := &Set{F: s.F}
  	result.once.Do(result.init)
  
  	for k, v := range s.m {
  		if _, ok := other.m[k]; !ok {
  			result.m[k] = v
  		}
  	}
  
  	return result
  }
docs: // Difference performs a set difference of the two sets, returning
  // a new third set that has only the elements unique to this set.