kotlin lazy 形式的 grouping and mapping 应该怎么写， sequence 里 grouping + aggregate 怎么比 Java stream 还复杂得多

是不是我姿势不对，有没有更好的写法？

val kotlin = "ABCDEF".asSequence()
    .groupingBy {
        println("grouping")
        it.toInt() / 10
    }
    .aggregate { _, accumulator: MutableList<String>?, element, first ->
        println("aggregate")
        if (first) {
            val tmp = MutableList(0) { "" }
            tmp.add(element.toString() + "X")
            tmp
        } else {
            accumulator!!.add(element.toString() + "X")
            accumulator
        }
    }
println(kotlin)


val java = "ABCDEF".toList().stream()
    .collect(
        Collectors.groupingBy(
            { println("java grouping"); it.toInt() / 10 },
            Collectors.mapping({ println("java mapping"); it.toString() + "X" }, Collectors.toList())
        )
    )
println(java)