code: func (r *run) updatePrimaryPhoto(photoNode *importer.Object) error {
  	photoId := photoNode.Attr(attrFlickrId)
  	for album, photo := range r.primaryPhoto {
  		if photoId != photo {
  			continue
  		}
  		setsNode, err := r.getTopLevelNode("sets", "Sets")
  		if err != nil {
  			return fmt.Errorf("could not set %v as primary photo of %v, no root sets: %v", photoId, album, err)
  		}
  		setNode, err := setsNode.ChildPathObject(album)
  		if err != nil {
  			return fmt.Errorf("could not set %v as primary photo of %v, no album: %v", photoId, album, err)
  		}
  		fileRef := photoNode.Attr(nodeattr.CamliContent)
  		if fileRef == "" {
  			return fmt.Errorf("could not set %v as primary photo of %v: fileRef of photo is unknown", photoId, album)
  		}
  		if err := setNode.SetAttr(nodeattr.CamliContentImage, fileRef); err != nil {
  			return fmt.Errorf("could not set %v as primary photo of %v: %v", photoId, album, err)
  		}
  		delete(r.primaryPhoto, album)
  	}
  	return nil
  }
docs: // updatePrimaryPhoto uses the camliContent of photoNode to set the
  // camliContentImage of any album for which photoNode is the primary photo.