It depends upon how big of a picture you want to look at. E.g. if you look down to the hardware level (your single ethernet cable connecting your computer to a

It depends upon how big of a picture you want to look at. E.g. if you look down to the hardware level (your single ethernet cable connecting your computer to a router) it is again no longer parallel (from what I recall - it's been a while since I've studied this). In the parallel gifts example, they had to somehow decide who would buy people what, which tends to not be very parallel. If the store has only one entrance then that too turns into a non-parallel part of the problem. If the gifts were mailed there's only 1 mailman who delivers to an area on a given day. And so on. You're probably always going to find resource contention somewhere, it just isn't always something you have to deal with directly when solving your problem.