The undo functionality is actually almost there, I think we just don't have an interface for it. The arrow keys thing should be easy to implement. I will add it to github, and of course accept a pull request! We definitely need to accept ranges for formulas or some up with some easy way to do that.

The project as a whole is definitely unfinished but we have been working on it for so long and lost some motivation. I wanted to just get it out there to at least re-motivate myself to work on it.