interface Populace {
   val people: Set<People>
}

interface Populace {

    val people : Set<People>
    fun getPopulation() : Int

}

data class People(val age:Int, val givenName:String, val surname:String)

class CompositePopulace(private val populi:Set<Populace>) : Populace {


    override val people by lazy {
        populi.flatMap { it.people }.toSet()
    }


    override fun getPopulation(): Int = people.size
}

population

interface Populace {

    val people : Set<People>
    val size : Int

}

data class People(val age:Int, val givenName:String, val surname:String)

class CompositePopulace(private val populi:Set<Populace>) : Populace {


    override val people by lazy {
        populi.map { it.people }.reduce { s,t -> s union t }
    }


    override val size = people.size
}