code: func (c *Corpus) GetParentDirs(ctx context.Context, childRef blob.Ref) (map[blob.Ref]struct{}, error) {
  	parents, ok := c.fileParents[childRef]
  	if !ok {
  		if _, ok := c.files[childRef]; !ok {
  			return nil, os.ErrNotExist
  		}
  		return nil, nil
  	}
  	return parents, nil
  }
docs: // GetParentDirs returns the direct parents (directories) of the file or directory childRef.
  // It only returns an error if childRef does not exist.