{"abstract":"The operation returns a result or retained state that violates this contract: Return unique union values in first appearance order across left then right.","category":"Collection algebra","checks":4,"contract":"Return unique union values in first appearance order across left then right. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","evaluation_group":"model-95dee12d632e234e","failed_approach":"Reversing the operands gives right-side encounters priority.","family":"xr-stable-set-union","id":"FA-4236","implementations":{"attempt":{"sha256":"c6f0de56aa810c4a1dfc6139a59a66afa5c1c24e5fcff936f34498eb4d6b3f17","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return list(dict.fromkeys(y+x))\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('both empty', solve([], []), [])\ncheck('empty right operand', solve(['a'], []), ['a'])\ncheck('empty left operand', solve([], ['b']), ['b'])\ncheck('overlap and multiplicity', solve(['b', 'a', 'b'], ['b', 'c', 'c']), ['b', 'a', 'c'])\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"072bec4d6cedb1fbf4af642ca69181657b6a67e8d249763bbaa5f40ea5e9e6e4","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return sorted(set(x+y))\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('both empty', solve([], []), [])\ncheck('empty right operand', solve(['a'], []), ['a'])\ncheck('empty left operand', solve([], ['b']), ['b'])\ncheck('overlap and multiplicity', solve(['b', 'a', 'b'], ['b', 'c', 'c']), ['b', 'a', 'c'])\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"fixed":{"sha256":"b264d3c25dc93cfc69a1c3fb1ba73a58a952148f94ac42e7b0dda23e070e39cb","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return list(dict.fromkeys(x+y))\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('both empty', solve([], []), [])\ncheck('empty right operand', solve(['a'], []), ['a'])\ncheck('empty left operand', solve([], ['b']), ['b'])\ncheck('overlap and multiplicity', solve(['b', 'a', 'b'], ['b', 'c', 'c']), ['b', 'a', 'c'])\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":" This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"xr-stable-set-union","generated_at":"2026-09-29T14:37:37.144465+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"A controlled local-runtime regression for collection APIs, language semantics, or ownership wrappers. Fixtures include boundary and interaction cases.","repair":"Return unique union values in first appearance order across left then right.","root_cause":"Sorting the union erases operand encounter order.","sha256":"139315f6b7c4c10d15a3e550bfc623de2f7cc5cd36e04538b9afbcfb5af96713","title":"Stable set union · case 01","variant":1,"variant_policy":"Five execution reruns of a fixed adversarial fixture suite; variant number does not alter semantic inputs.","verification":{"attempt":{"elapsed_ms":39.329,"exit_code":1,"observations":[{"actual":[],"check":"both empty","expected":[],"passed":true},{"actual":["a"],"check":"empty right operand","expected":["a"],"passed":true},{"actual":["b"],"check":"empty left operand","expected":["b"],"passed":true},{"actual":["b","c","a"],"check":"overlap and multiplicity","expected":["b","a","c"],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"both empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"empty right operand\", \"actual\": [\"a\"], \"expected\": [\"a\"], \"passed\": true}, {\"check\": \"empty left operand\", \"actual\": [\"b\"], \"expected\": [\"b\"], \"passed\": true}, {\"check\": \"overlap and multiplicity\", \"actual\": [\"b\", \"c\", \"a\"], \"expected\": [\"b\", \"a\", \"c\"], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":37.398,"exit_code":1,"observations":[{"actual":[],"check":"both empty","expected":[],"passed":true},{"actual":["a"],"check":"empty right operand","expected":["a"],"passed":true},{"actual":["b"],"check":"empty left operand","expected":["b"],"passed":true},{"actual":["a","b","c"],"check":"overlap and multiplicity","expected":["b","a","c"],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"both empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"empty right operand\", \"actual\": [\"a\"], \"expected\": [\"a\"], \"passed\": true}, {\"check\": \"empty left operand\", \"actual\": [\"b\"], \"expected\": [\"b\"], \"passed\": true}, {\"check\": \"overlap and multiplicity\", \"actual\": [\"a\", \"b\", \"c\"], \"expected\": [\"b\", \"a\", \"c\"], \"passed\": false}], \"passed\": false}\n"},"fixed":{"elapsed_ms":38.483,"exit_code":0,"observations":[{"actual":[],"check":"both empty","expected":[],"passed":true},{"actual":["a"],"check":"empty right operand","expected":["a"],"passed":true},{"actual":["b"],"check":"empty left operand","expected":["b"],"passed":true},{"actual":["b","a","c"],"check":"overlap and multiplicity","expected":["b","a","c"],"passed":true}],"passed":true,"stderr":"","stdout":"{\"observations\": [{\"check\": \"both empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"empty right operand\", \"actual\": [\"a\"], \"expected\": [\"a\"], \"passed\": true}, {\"check\": \"empty left operand\", \"actual\": [\"b\"], \"expected\": [\"b\"], \"passed\": true}, {\"check\": \"overlap and multiplicity\", \"actual\": [\"b\", \"a\", \"c\"], \"expected\": [\"b\", \"a\", \"c\"], \"passed\": true}], \"passed\": true}\n"}},"verified":true,"visibility":"public"}