It's used for younger kids. However, if I recall correctly, I had a box (a small empty square) or sometimes a blank space denoted by an underscore. I guess they think it's easier for them to understand something that represents blank rather than 'x'. It's to introduce algebra concepts to kids before they're in algebra (maybe >5 years before).

You're confusing two different issues. Some (many) models have having an accelerator problem. Supposedly, the car takes off and there's no way to stop it.

Then, there's the brake issue with the Prius. If you press on the brake lightly, it only uses the regenerative braking (electric). If you hit a pothole, the ABS kicks in and there's a switchover to the friction brakes. You temporarily lose some braking force and it feels like the car is floating or (as some have reported) accelerating.

I own the affected Prius model. I've experienced the issue and I don't think it's a problem. It was a little unnerving until I realized what it was. If I really need to stop sooner when the brakes "fail", all I have to do is hit the pedal harder and it does what I expect.

I tried to compile it, but it's missing TravelerDiskWizard.h. I was really looking forward to playing with this thing...