code: func HashReq(req *chartutil.Requirements) (string, error) {
  	data, err := json.Marshal(req)
  	if err != nil {
  		return "", err
  	}
  	s, err := provenance.Digest(bytes.NewBuffer(data))
  	return "sha256:" + s, err
  }
docs: // HashReq generates a hash of the requirements.
  //
  // This should be used only to compare against another hash generated by this
  // function.