code: func (m *InMem) Prune(ctx context.Context, prefix string) error {
  	m.Lock()
  	defer m.Unlock()
  
  	deleted := 0
  	for k := range m.data {
  		if strings.HasPrefix(k, prefix+"/") {
  			delete(m.data, k)
  			deleted++
  		}
  	}
  	if deleted < 1 {
  		return fmt.Errorf("not found")
  	}
  	return nil
  }
docs: // Prune removes a directory