Some background on the kind of person Phil Bagwell was and his contributions http://blog.fogus.me/2012/10/15/phil-bagwell-rest-in-peace

Of particular note is his work creating Hash Array Mapped Tries (aka functional associative arrays) detailed in http://lampwww.epfl.ch/papers/idealhashtrees.pdf and implemented in Haskell, Clojure, Scala and Rubinius. His latest work involved lock-free versions called CTrees[1] that are likely to create more waves as others become familiar with them.

[1]: https://github.com/axel22/Ctries