I agree that's an optimization and an important litmus test, but calling any TCO that doesn't also handle mutual recursion (but does naive fib just fine) not "r

I agree that's an optimization and an important litmus test, but calling any TCO that doesn't also handle mutual recursion (but does naive fib just fine) not "real" feels like a stretch. At least we agree on the facts: if you want mutual recursion in Clojure you need to write it with letfn + trampoline. That feels like it's only a hop away from recur to me, as in: if recur doesn't feel like real TCO to you I imagine letfn + trampoline won't either, but letfn+trampoline means that I can write all the mutual recursion things I want to write almost exactly the way I want to write them, so it's close enough for me.