Tap a cot, then the couple you're assigning it to. Only one arrangement doesn't contradict any law of inheritance.
When the lights came back on, all three identification bracelets were lying on the floor. Nobody remembers which baby was in which cot, and the three mothers have been in the corridor since dawn. All there is to go on is the delivery book, with the blood group of the six parents, and this morning's analysis of the three newborns.
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