Kotlin Multi-Platform - What's the Hardest Part?

Kotlin has this language support that says that in your shared piece of Kotlin code, you can say, I expect that the platforms that I'm going to target will provide an actual implementation. And so it's kind of like an interface where you define the inner, the common interface. And then the specific platforms define the implementations. So if you're targeting Android, then you're going to have an actual for Android. If you're targeting iOS, you'regoing to have anactual for iOS and so on and so forth.