code: func Decode(payload string) (*ClaimSet, error) {
  	// decode returned id token to get expiry
  	s := strings.Split(payload, ".")
  	if len(s) < 2 {
  		// TODO(jbd): Provide more context about the error.
  		return nil, errors.New("jws: invalid token received")
  	}
  	decoded, err := base64Decode(s[1])
  	if err != nil {
  		return nil, err
  	}
  	c := &ClaimSet{}
  	err = json.NewDecoder(bytes.NewBuffer(decoded)).Decode(c)
  	return c, err
  }
docs: // Decode decodes a claim set from a JWS payload.