[aatd86]

Ah I see. Why throw exceptions? They could just be noop depending on the typestate?

But still it doesn't look very nice in practice. I think that in that case, it does make much more sense for a mutable type to be a subtype of the immutable one.

The immutable threw me off track. It's just that the supertype would lack mutation operations. (in a sense immutable would be the default for every type).

Thank you.