sealed class Option<out T> {
    object None : Option<Nothing>()
    data class Some<out T>(val value: T) : Option<T>()
}

sealed class Either<out A, out B> {
    data class Left<out A>(val value: A) : Either<A, Nothing>()
    data class Right<out B>(val value: B) : Either<Nothing, B>()
}

sealed class List<out A> {
    object Nil : List<Nothing>()
    data class Cons<out A>(val head: A, val tail: List<A>) : List<A>()
}