Sixteen federal states, with one split into two, gives 17 chambers, the same lines the profession already organises itself along, which means the model needed no explaining to the people inside it. Within a chamber, vets sit in clusters small enough that a night's cover is a local arrangement rather than a national lottery.
Fair allocation runs inside the cluster first. Only the resolved plan is published out to devices, so a vet's phone never shows a provisional rota that might change under them, the single biggest source of distrust in a shared-duty system.