I have solved the puzzle that you are pointing me to. But, when I click on Run button (bellow the Ruby REPL), nothing happens. In the developer tools, I see an error being logged in the console:

api.js:2 Unhandled rejection TypeError: Cannot set property 'onclose' of null at https://repl.it/lib/api.js:1:9685 at tryCatcher (https://repl.it/lib/api.js:4:24715) at Promise._settlePromiseFromHandler (https://repl.it/lib/api.js:3:11737) at Promise._settlePromiseAt (https://repl.it/lib/api.js:3:13453) at Promise._settlePromises (https://repl.it/lib/api.js:3:15793) at Async._drainQueue (https://repl.it/lib/api.js:1:17849) at Async._drainQueues (https://repl.it/lib/api.js:1:17994) at Async.drainQueues (https://repl.it/lib/api.js:1:15632)