Dual-encode one live tax-benefit instrument — French allocations familiales, or UK Universal Credit — in Catala and in OpenFisca or PolicyEngine, against the same household file. Publish disagreements as bugs versus documented modeling choices (benefit unit, take-up, labor-supply elasticity, CGE closure), and test whether a named reform's ranking flips when only those knobs move. If it does not, "compute, then vote the leftover" is enough in that domain. If it does, the first product to ship is a ledger of encodings, not a single feasible set.