kotlin エラー「error: class ‘xxx’ is not abstract and does not implement abstract base class member public abstract fun x(): Unit defined in xxx」の解決方法

abstract class Oya {

    abstract fun f()
    abstract fun m() 

}

class Ko() : Oya() {

    override fun f() { println("hello") }

}

fun main(){

    val ko = Ko()
    ko.f() // hello

}

error: class 'Ko' is not abstract and does not implement abstract base class member public abstract fun m(): Unit defined in Oya
class Ko() : Oya() {

abstract class Oya {

    abstract fun f()
    abstract fun m() 

}

class Ko() : Oya() {

    override fun f() { println("hello") }
    override fun m() { println("world") }

}

fun main(){

    val ko = Ko()
    ko.f() // hello

}