code: func SetURL(urls ...string) ClientOptionFunc {
  	return func(c *Client) error {
  		switch len(urls) {
  		case 0:
  			c.urls = []string{DefaultURL}
  		default:
  			c.urls = urls
  		}
  		return nil
  	}
  }
docs: // SetURL defines the URL endpoints of the Elasticsearch nodes. Notice that
  // when sniffing is enabled, these URLs are used to initially sniff the
  // cluster on startup.