An immutable set based on a red-black tree. Adapted from Okasaki (1998) and Germane and Might (2014).
Germane, K. and Might, M. (July 2014). "Deletion: The curse of the red-black tree". In: Journal of Functional Programming 24, pp. 423–433.
Okasaki, C. (1998). Purely Functional Data Structures. Cambridge University Press.