code: func (c *Corpus) GetDirChildren(ctx context.Context, dirRef blob.Ref) (map[blob.Ref]struct{}, error) {
  	children, ok := c.dirChildren[dirRef]
  	if !ok {
  		if _, ok := c.files[dirRef]; !ok {
  			return nil, os.ErrNotExist
  		}
  		return nil, nil
  	}
  	return children, nil
  }
docs: // GetDirChildren returns the direct children (static-set entries) of the directory dirRef.
  // It only returns an error if dirRef does not exist.