code: func (mgr *MemorySessionManager) NewSession() (Session, error) {
  	sid, err := mgr.sessionID()
  	if err != nil {
  		return nil, err
  	}
  	s := MemorySession{sid: sid, data: make(map[string]interface{}), createdAt: time.Now()}
  	mgr.lock.Lock()
  	mgr.sessions[sid] = &s
  	mgr.lock.Unlock()
  	return &s, nil
  }
docs: // NewSession creates and returns a new session.