My point is that adding TCO is not a breaking semantics change, as its only visibility will be through a) the fact that more code works with it than w/o it, b)

My point is that adding TCO is not a breaking semantics change, as its only visibility will be through a) the fact that more code works with it than w/o it, b) elision of tail-calling frames in stack traces. In particular I don't consider (b) to be a problem. I'm aware (b) is a matter of opinion, but really, TCO is much too valuable to use (b) as an argument against TCO.