code: func (m *Memory) Get(ctx context.Context, key []byte) (*backend.Item, error) {
  	if len(key) == 0 {
  		return nil, trace.BadParameter("missing parameter key")
  	}
  	m.Lock()
  	defer m.Unlock()
  	m.removeExpired()
  	i := m.tree.Get(&btreeItem{Item: backend.Item{Key: key}})
  	if i == nil {
  		return nil, trace.NotFound("key %q is not found", string(key))
  	}
  	item := i.(*btreeItem).Item
  	return &item, nil
  }
docs: // Get returns a single item or not found error