immutable.kt

sealed class MyValue(default: String) {

   open val value: String = default

   class Immutable(default: String): MyValue(default) {
       fun mutate(): Mutable = Mutable(value)
   }

   class Mutable(default: String) : MyValue(default) {
       fun immutate(): Immutable = Immutable(value)

       override var value = default
   }
}