class RepoElement<E>(
    val elem: E,
    val version: Long
)
class ListRepo<E> : Repo<E> {
    private val list = ConcurrentHashMap<String, RepoElement<E>>()
    private fun version() = System.currentTimeMillis()
    
    override fun create(element: E): Boolean =
        true.apply {
            list[UUID.randomUUID().toString()] =
                RepoElement(element, version())
        }

override fun read(): List<Item<E>> =
    list.map {
        Item(it.value.elem, it.key, it.value.version)
    }

override fun update(item: Item<E>): Boolean =
    list[item.id]?.let {
        if (it.version==item.version){
            list[item.id]= RepoElement(item.elem, version())
            true
        } else {
            false
        }
    }?: false

put {
    val newItem = 
        Json.decodeFromString(itemSerializer, call.receive())
    repo.read(newItem.id) ?: return@put ...
    if(repo.update(newItem)) {
        call.respondText(
            "Element updates correctly",
            status = HttpStatusCode.Created)
        } else {
        call.respondText(
            "Element had updated on server",
            status = HttpStatusCode.BadRequest)
    }
}

post(AddStudentToLesson.path) {
    val command = Json.decodeFromString(
        AddStudentToLesson.serializer(), call.receive())
    val lesson = lessonsRepo.read ...
    studentsRepo.read ...
    if (lesson.elem.students.find { 
        it.studentId == command.lessonId } != null) ...
    if (command.version != lesson.version) {
        call.respondText(
            "Lesson had updated on server",
            status = HttpStatusCode.BadRequest)}
    val newLesson = lesson.elem.addStudent(command.studentId)
    ...