I would also recommend Koen Claessen's simplified finger trees (https://dl.acm.org/doi/abs/10.1145/3406088.3409026 https://dl.acm.org/doi/abs/10.1145/3406088.34

I would also recommend Koen Claessen's simplified finger trees (https://dl.acm.org/doi/abs/10.1145/3406088.3409026 https://dl.acm.org/doi/abs/10.1145/3406088.3409026) and Zip trees (https://arxiv.org/pdf/1806.06726.pdf https://arxiv.org/pdf/1806.06726.pdf) for purely functional skip lists.