> the handoff can fail in the same way Everything in a digital system can fail, but by convention (and because it's not so far from the actual truth) some part

> the handoff can fail in the same way Everything in a digital system can fail, but by convention (and because it's not so far from the actual truth) some parts are assumed to be 100% reliable when modeling them. If you don't make this assumption, you can't guarantee anything.