What's the actual data structure behind this? Is it something like the blist[1] (i.e., counted b+ tree)?

[1] https://pypi.python.org/pypi/blist/

EDIT: Nvm, the article actually links to the implementation and it seems to be an order statistics 2-3 tree: http://www.opensource.apple.com/source/CF/CF-368.18/Collecti...