There's a really simple model. No callbacks. Just, it's like at this moment, I want to materialize this variable and I want to let the user select it. And if they do change it, what happens is the app runs from top to bottom. At that exact moment when that call happens, we substitute 50 for whatever number the user typed in. So the function is basically rerun. It's sort of been seeded to say, if they ask, tell them it was this.