val heroes = listOf(
  Hero("The Captain", 60, MALE),
  Hero("Frenchy", 42, MALE),
  Hero("The Kid", 9, null),
  Hero("Lady Lauren", 29, FEMALE),
  Hero("First Mate", 29, MALE),
  Hero("Sir Stephen", 37, MALE))

val mapByAge: Map<Int, List<Hero>> =
  heroes.groupBy { it.age }
val (age, group) = mapByAge.maxBy { (_, group) ->
  group.size
}!!
println(age)