[marijn]

I am aware of those, but consciously chose not to use them. A hierarchical document model, as opposed to their linear one, has a lot of advantages, and OT is a pain to extend. See also http://marijnhaverbeke.nl/blog/collaborative-editing.html

[nornagon]

The JSON data type in ShareJS is not linear, though that said implementing rich text on top of it is still an open problem.

Full disclosure: I wrote the JSON data type in ShareJS :)