class Nesting (val parent: Nesting? = null) {
    val map = mutableMapOf("id" to nextId ())
    override fun toString(): String = "${map["id"]}"

    companion object {
        private var count = 0
        fun nextId () = count ++
    }
}

fun main () {
    val a = Nesting (Nesting (Nesting ()))

    println (a.map)
    println (a.parent?.map.get ("id") ?: "?")
}

?.

a?.b

(a ?: return null).b

?.

?.

.