look at the signature for `with`

    inline fun <T, R> with(receiver: T, f: T.() -> R): R 

it takes a lambda that is scoped to the receiver type `T`. This means the lambda runs in the scope of whatever is passed in as a receiver.

    with(list) {
      add(item)
      add(item)
    }

This essentially lets you create DSLs for whatever.

Another thing that's bit me in butt recently is that `Protocol`s aren't concrete types, unlike an `interface` in Java/Kotlin.