For those like me who are unfamiliar with Relaxed Radix Balanced trees, here's the paper presenting the algorithm:

RRB-Trees: Efficient Immutable Vectors (2012)

https://infoscience.epfl.ch/record/169879/files/RMTrees.pdf

EDIT: For further reading:

Improving RRB-Tree Performance through Transience (2014)

https://hypirion.com/thesis.pdf

Article: https://hypirion.com/musings/thesis

Code: https://github.com/hyPiRion/c-rrb