code: func (mgr *MemorySessionManager) Session(sid string) (Session, error) {
  	mgr.lock.RLock()
  	if s, ok := mgr.sessions[sid]; ok {
  		s.createdAt = time.Now()
  		mgr.lock.RUnlock()
  		return s, nil
  	}
  	mgr.lock.RUnlock()
  	return nil, fmt.Errorf("Can not retrieve session with id %s", sid)
  }
docs: // Session gets the session instance by indicating a session id.