code: func (s *Set) Difference(other *Set) *Set {
  	result := new(Set)
  	if s != nil {
  		for k, v := range s.m {
  			var ok bool
  			if other != nil {
  				_, ok = other.m[k]
  			}
  			if !ok {
  				result.Add(v)
  			}
  		}
  	}
  
  	return result
  }
docs: // Difference returns a set with the elements that s has but
  // other doesn't.