code: func (s Store) Get(key string) (KVPair, error) {
  	s.RLock()
  	kv, ok := s.m[key]
  	s.RUnlock()
  	if !ok {
  		return kv, &KeyError{key, ErrNotExist}
  	}
  	return kv, nil
  }
docs: // Get gets the KVPair associated with key. If there is no KVPair
  // associated with key, Get returns KVPair{}, ErrNotExist.