The Scala/Clojure approach for persistent arrays works great with our
`reset/reuse`. We seem to be much more efficient than their
implementations because of `reset/reuse`. The new approach also seems
better than the old one implemented in the runtime, and has a few
advantages:
1- The reroot procedure used in the old approach required
synchronization for multi-threaded code, or we would need to perform
deep copies when sending `parray` objects between threads.
2- We don't need any runtime extension for the new approach.
3- The old approach used "trail lists" for undoing array updates.
This works well for bactracking search use cases, but it is bad
in use cases where we are simultaneously updating the persistent
arrays that have shared nodes.
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