kotlin sealed class

sealed interface Expr

sealed class MathExpr(): Expr

data class Const(val number: Double) : MathExpr()

data class Sum(val e1: Expr, val e2: Expr) : MathExpr()

object NotANumber : Expr

sealed class Message {
    abstract val messageId: String
}
data class Track(val event: String, override val messageId: String): Message()