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Not so much a thought experiment as a single neuron misfire.
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A kit that enables you to make any size or length cord/wire retractable.
 alright... by supplying a number of different cylindrical spool heights, diameters with exchangeable, different strength springs (if that is how they work) specified for different types of wires, that you can look up in
the manual that comes with the kit.
||Forty or so years ago, before seatbelts were built-in, and automatically retracted, one could buy seat-belt retractors as an add-on. These were much as you describe, spring loaded spools.