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(pairlisto l1 l2 pairs): pairs is the zipped list of pairs (l1[i] l2[i]). Recurses on both l1 and l2 in lockstep, building pairs in parallel. Runs forward, can recover l1 given l2 and pairs, can recover l2 given l1 and pairs. Different-length lists fail. 5 new tests, 431/431 cumulative.