code: func (conn *Conn) Cap(subcommmand string, capabilities ...string) {
  	if len(capabilities) == 0 {
  		conn.Raw(CAP + " " + subcommmand)
  	} else {
  		conn.Raw(CAP + " " + subcommmand + " :" + strings.Join(capabilities, " "))
  	}
  }
docs: // Cap sends a CAP command to the server.
  //     CAP subcommand
  //     CAP subcommand :message