code: func (t *Token) WithExtra(extra interface{}) *Token {
  	t2 := new(Token)
  	*t2 = *t
  	t2.raw = extra
  	return t2
  }
docs: // WithExtra returns a new Token that's a clone of t, but using the
  // provided raw extra map. This is only intended for use by packages
  // implementing derivative OAuth2 flows.