class Foo { 
    operator fun invoke(fooBlock: Foo.() -> Unit) =
        this.apply(fooBlock)
    
    operator fun String.invoke(barBlock: Bar.() -> Unit) = 
        Bar().apply(barBlock)
        
    fun sayFoo() {
        println("I'm a Foo!")
    }
}

class Bar {
    operator fun invoke(barBlock: Bar.() -> Unit) =
        this.apply(barBlock)
        
    fun sayBar() {
        println("I'm a Bar!")
    }
}

fun main() {
    val foo = Foo()
    
    foo {
        sayFoo()
        "and there should be a bar block" {
            sayBar()
            "this shouldn't be allowed I think" {
                sayFoo()
                sayBar()
            }
        }
    }
}