There's many physical examples of systems like this as well. Take e.g. a redundant interlock system that disallows opening a chassis unless the power is off an

There's many physical examples of systems like this as well. Take e.g. a redundant interlock system that disallows opening a chassis unless the power is off and you've inserted a special key. Useless on a juice press #cough#juiceroo#cough#, but very important on a 20 W UV laser. Or a six-point safety harness. I'm not going to put them in my daily driver, but I wouldn't get into a racecar without them.