I would actually have preferred that the old mux was left as is, and that the new mux had been a new package. This would have eliminated the need to be backward

I would actually have preferred that the old mux was left as is, and that the new mux had been a new package. This would have eliminated the need to be backwards compatible and allowed more freedom in creating a better API. I think that putting the method in the matching expression is a step back as it can't make use of the compiler to enforce correctness.