A 5 cannot crash into an 8. But if a 5 crashes into a 4, it becomes a 9.
This is problematic when you've got a 1 that must merge with a 9 in another sector.
I was not expecting much from 10. However, I am much impressed.
10 from iojoe is challenging and fun. There are some 140 puzzle levels. Right now I've done about 100 of them. It's a winner. And it's free too! At least for now. So hurry up and download it. The graphics are good too.