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kotlinlang

Okay, Something has been bothering me for a few days now. I'm wondering If Kotlin could develop features (specially in the JVM) in the future or not.
Could we add a Interface to a class with an extension?
For example we have a class `Test` (that is in a library) and we don't have access to it's code, and we have a interface like `Killer`, Can we make `Test` implement `Killer` with an extension? this way wherever `Test` is used, we have access to `Killer` functions.
The solution in my head is that Kotlin can do this by creating a `Test$Killer` class that extends `Test` and replace `Test` with `Test$Killer` everywhere in the code. But this only works for open classes.
This can be done for final classes in Swift, and I'm wondering if that could be done without modifying the bytecode.