code: func (h *handler) fetchDocument(pn blob.Ref) (*document, error) {
  	return h.fetchDocumentFunc([]blob.Ref{pn}, func(blobs []blob.Ref) (*search.SearchResult, error) {
  		return h.searchDocument(context.TODO(), blobs[0])
  	})
  }
docs: // returns os.ErrNotExist when document was not found