class Test {
    var a = "a"
        set(value) {
            field = checkValue(field, value)
        }
    var b = "b"
        set(value) {
            field = checkValue(field, value)
        }

    private fun checkValue(current: String, new: String): String {
        return if (new > current) new else current
    }
}

class GreatestDelegate<T: Comparable<T>>(initialValue: T) {
    private var backingField: T = initialValue

    operator fun getValue(host: Any?, property: KProperty<*>): T  {
        return backingField
    }

    operator fun setValue(host: Any?, property: KProperty<*>, value: T) {
        if (value > backingField) {
            backingField = value
        }
    }
}

class Test {
    var a: String by GreatestDelegate("")
    var b: String by GreatestDelegate("_")
}