Create sealed-hierarchies-and-when.md

spec-docs/sealed-hierarchies-and-when.md

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+# Sealed Class Hierarchies and when-expressions
+
+Goals:
+* Make sealed class hierarchies possible (this implies more exhaustiveness criteria for `when`-expressions)
+
+## Discussion
+
+One option would be to make `when` understand hierarchies like this one:
+
+``` kotlin
+abstract class Type private () {
+    class Named(val name: String) : Type()
+    class Function(val param: Type, val result: Type): Type()
+    object Top: Type()
+}
+```
+
+When the superclass has a private constructor, all its subclasses are known to be (directly or indirectly) nested into in, 
+and we can be sure that a set of `is`-checks is exhaustive if all the subclasses are included:
+
+``` kotlin
+when (type) {
+    is Named -> println(name)
+    is Function -> println("$param -> $result")
+    is Top -> println("TOP")
+}
+
+```
+