code: func paramsForMessageList(params *ListParams) (*url.Values, error) {
  	urlParams := &url.Values{}
  
  	if params == nil {
  		return urlParams, nil
  	}
  
  	if params.Direction != "" {
  		urlParams.Set("direction", params.Direction)
  	}
  	if params.Originator != "" {
  		urlParams.Set("originator", params.Originator)
  	}
  	if params.Limit != 0 {
  		urlParams.Set("limit", strconv.Itoa(params.Limit))
  	}
  	urlParams.Set("offset", strconv.Itoa(params.Offset))
  
  	return urlParams, nil
  }
docs: // paramsForMessageList converts the specified MessageListParams struct to a
  // url.Values pointer and returns it.