code: func (r *Session) AppendUserAgent(agent string) {
  	if r.userAgent == "" {
  		r.userAgent = getDefaultUserAgent()
  	}
  
  	if agent != "" {
  		r.userAgent += " " + agent
  	}
  }
docs: // AppendUserAgent allows higher level application to identify themselves by
  // appending to the useragent string