code: func (s *Cache) removeExpired() raiqub.LockStatus {
  	writeLocked := false
  	s.RLock()
  	for i := range s.values {
  		if s.values[i].IsExpired() {
  			if !writeLocked {
  				s.RUnlock()
  				s.Lock()
  				writeLocked = true
  			}
  			delete(s.values, i)
  		}
  	}
  
  	if writeLocked {
  		return raiqub.WriteLocked
  	} else {
  		return raiqub.ReadLocked
  	}
  }
docs: // removeExpired remove all expired values from current Cache instance list.
  // Returns the locking status of current instance.