Kotlin: sealed class cannot "contain" data classes? Why?

OK, now that Kotlin is officially out and I am starting to play with it again, I am quite confused that I need to choose between the advantages of sealed and data but somehow can't have both.

This, for example, seems to make sense to me, but does not compile:

sealed class Expr {
    data class Const(val number: Double) : Expr()
    data class Sum(val expr1 : Expr, val expr2 : Expr) : Expr()
}

because the data classes cannot extend other classes.

Is there something I am missing?