6to5 now rewrites functions with TCO! https://github.com/6to5/6to5/pull/714#issuecomment-73433079 (I wrote an elisp equivalent, but this is far more general)
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Interesting JVM JIT example where the author rewrites code to help the optimiser: http://t.co/9sNIe7d59l (ideally we'd have the reverse!)