public inline fun <T> Iterable<T>.sumBu(selector: (T) -> Int): Int {
var sum: Int = 0
for (element in this) {
sum += selector(element)
}
return sum
}
val list = listOf(Person("a", 10), Person("b", 20), Person("c", 30))
println(list.sumBy{ it.age }) // 55
