code: func (s *Sublist) addToCache(subject string, element interface{}) {
  	for k, r := range s.cache {
  		if matchLiteral(k, subject) {
  			// Copy since others may have a reference.
  			nr := append([]interface{}(nil), r...)
  			nr = append(nr, element)
  			s.cache[k] = nr
  		}
  	}
  }
docs: // addToCache will add the new entry to existing cache
  // entries if needed. Assumes write lock is held.