IBookService.kt

interface IBookService {

  fun findBooks(user: User, from: LocalDate, to: LocalDate): List<Tuple2<LocalDate, Book>>

  fun findUserBooks(users: List<User>, from: LocalDate, to: LocalDate): List<Tuple3<LocalDate, User, Book>> {
    return users.flatMap { user ->
      findBooks(user , from , to).map { it -> Tuple3(it.v1 , user , it.v2) }
    }.sortedBy { triple -> triple.v1 }
  }
}