fun Node.removedWhen(expr: () -> ObservableValue<Boolean>) {
    val originalParent = parent
    val placeholder = Region()

    fun remove() {
        val index = Math.max(0, originalParent.childrenUnmodifiable.indexOf(this))
        removeFromParent()
        originalParent.addChildIfPossible(placeholder, index)
    }

    fun add() {
        val index = Math.max(0, originalParent.childrenUnmodifiable.indexOf(placeholder))
        removeFromParent()
        originalParent.addChildIfPossible(this, index)
    }

    fun op(remove: Boolean) = if (remove) remove() else add()

    val state = expr()
    
    state.onChange {
        op(it ?: false)
    }

    op(state.value)
}