code: func (set *Set) Flatten() []interface{} {
  	set.lock.Lock()
  	defer set.lock.Unlock()
  
  	if set.flattened != nil {
  		return set.flattened
  	}
  
  	set.flattened = make([]interface{}, 0, len(set.items))
  	for item := range set.items {
  		set.flattened = append(set.flattened, item)
  	}
  	return set.flattened
  }
docs: // Flatten will return a list of the items in the set.