Haskell's `fingertree` library has a really nice [search](https://hackage.haskell.org/package/fingertree-0.1.4.2/docs/Data-FingerTree.html#v:search) function. It is useful for building a split function for ropes. I have a purescript implementation [here](https://github.com/ssbothwell/text-editor-prototype/blob/main/src/Data/FingerTree/Search.purs). Would you be interested in a PR to bring it into `purescript-sequences`?
