rts is the mechanism we use to send values between JavaScript and Gren _without_ sacrificing\\nGren's guarantee of immutability and managed side effects.\\n\\nOne limitation of ports is that you can only define `Cmd` (sending a message from Gren to JavaScript) and `Sub` (JavaScript to Gren) ports.\\nIf you wanted to send a value to JavaScript and immediately get a response, you'd have to model that with two ports.\\n\\nAs an example, if you wanted to use JavaScript to store a value in a database and get the