{"abstract":"The operation returns a result or retained state that violates this contract: Return left-only then right-only unique values in their respective first-appearance orders.","category":"Collection algebra","checks":4,"contract":"Return left-only then right-only unique values in their respective first-appearance orders. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","evaluation_group":"model-22a52e0260840c6b","failed_approach":"Computing only the left difference omits right-only members.","family":"xr-stable-symmetric-set-difference","id":"FA-4261","implementations":{"attempt":{"sha256":"994ae2cdaa4ca14a11d39e24bb7c640e2d4fb5837575b0a5e7b0e8ae3d89a83d","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return [v for v in dict.fromkeys(x) if v not in 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']), ['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":"02ace3d9906de362b4b4a5a00b9aef07ef871de665395ab9df3986d66c029a53","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']), ['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":"cd54218bbf6245f857f2d9eb6b95e10e546417a290d185277acc5a0a2b90eef9","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return [v for v in dict.fromkeys(x+y) if (v in x)!=(v in 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']), ['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-symmetric-set-difference","generated_at":"2026-09-29T14:37:37.219038+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 left-only then right-only unique values in their respective first-appearance orders.","root_cause":"Union membership retains shared values that should be excluded.","sha256":"ae7e82f90580ad4bbc0a5f25539ea8c761c1d56856bcf914f3914524b9e8bbcc","title":"Stable symmetric set difference · 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":37.053,"exit_code":1,"observations":[{"actual":[],"check":"both empty","expected":[],"passed":true},{"actual":["a"],"check":"empty right operand","expected":["a"],"passed":true},{"actual":[],"check":"empty left operand","expected":["b"],"passed":false},{"actual":["a"],"check":"overlap and multiplicity","expected":["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\": [], \"expected\": [\"b\"], \"passed\": false}, {\"check\": \"overlap and multiplicity\", \"actual\": [\"a\"], \"expected\": [\"a\", \"c\"], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":35.848,"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","a","c"],"check":"overlap and multiplicity","expected":["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\", \"a\", \"c\"], \"expected\": [\"a\", \"c\"], \"passed\": false}], \"passed\": false}\n"},"fixed":{"elapsed_ms":37.829,"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":["a","c"],"check":"overlap and multiplicity","expected":["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\": [\"a\", \"c\"], \"expected\": [\"a\", \"c\"], \"passed\": true}], \"passed\": true}\n"}},"verified":true,"visibility":"public"}