add @aaronj1335's solution for doublets

s/o to @mwfogleman for [the idea to use tree-seq][the setup tho].

[the setup tho]: https://mwfogleman.github.io/posts/20-12-2014-flatcat.html
Merge pull request #10 from aaronj1335/doublify

add @aaronj1335's solution for doublets