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memory footprint reduced with a factor of 15 (wait what? Was the old implementation so ineffective? Or the new one is so cool? The truth is: both. Check out benchmarks)
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changes in API, most notably PersistentMap -> PMap, PersistentVector -> PVec
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more effective == and != on PMap
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deleted several classes, the whole class/interface hierarchy becomes much simpler (although little bit dirtier; some performance-motivated compromises were introduced)