Why fork (and reverse half of Coco's semantics), instead of contributing to Coco?

Do you expect that any of your additions will get merged into mainline Coco/CoffeeScript?

Most of my changes would be unacceptable to Satyr, the maintainer of Coco. For instance, I posted an issue about creating proper list comprehensions in both CoffeeScript and Coco, and both times was shut down. That's why I decided to create my own project, and make the changes I want without being shut down all the time. I don't expect any of my changes to be merged into Coco or CoffeeScript, but if they want to then I would encourage them to do so.