Really?! Sorry you were hurt, glad you're not dead. BUT, how does anyone end up working on a starter on a gear tractor, with the tractor in gear, without disconnecting the battery negative post's cable?
This kind of 'accident' is totally preventable with just the bare minimum of common sense, and safety precautions in place before doing any wrenching....
BTW, those who disconnect seat switches and similar safety devices should get what they deserve when/if things go wrong, and hopefully not end up hurting an innocent bystander in the process.