{"abstract":"The operation returns a result or retained state that violates this contract: Return the penultimate element, or None for lengths zero and one.","category":"Sequence semantics","checks":4,"contract":"Return the penultimate element, or None for lengths zero and one. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","contract_signature":"x, y=None","evaluation_group":"model-c5cbed0b742c8361","failed_approach":"Index one selects the second element, not the penultimate element of longer lists.","family":"xr-penultimate-element","id":"FA-4131","implementations":{"attempt":{"sha256":"bd5281aefe95baca69cc1231a30af5598d400f4299ee56eebace8c7e4a48bb5f","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return x[1] if len(x)>1 else None\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('empty collection', solve([]), None)\ncheck('singleton collection', solve(['a']), None)\ncheck('two distinct elements', solve(['a', 'b']), 'a')\ncheck('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), 'b')\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":"2489f6479c1e51a11e189a42c202868cfc2f30e7ac79944abe6b05e158f20211","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return x[-1] if x else None\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('empty collection', solve([]), None)\ncheck('singleton collection', solve(['a']), None)\ncheck('two distinct elements', solve(['a', 'b']), 'a')\ncheck('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), 'b')\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-penultimate-element","generated_at":"2026-09-29T14:37:35.951385+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.","root_cause":"The final element is selected rather than the preceding position.","sha256":"4ea0e9be2d8452dd1cf74748ab8382fc9e057429eba723cd67a0201d8b34bba3","title":"Penultimate element · case 01","variant":1,"variant_policy":"Five execution reruns of a fixed adversarial fixture suite; variant number does not alter semantic inputs.","verified":true,"visibility":"public","verification":{"attempt":{"elapsed_ms":39.909,"exit_code":1,"observations":[{"actual":null,"check":"empty collection","expected":null,"passed":true},{"actual":null,"check":"singleton collection","expected":null,"passed":true},{"actual":"b","check":"two distinct elements","expected":"a","passed":false},{"actual":"a","check":"nonadjacent duplicate and unsorted order","expected":"b","passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"empty collection\", \"actual\": null, \"expected\": null, \"passed\": true}, {\"check\": \"singleton collection\", \"actual\": null, \"expected\": null, \"passed\": true}, {\"check\": \"two distinct elements\", \"actual\": \"b\", \"expected\": \"a\", \"passed\": false}, {\"check\": \"nonadjacent duplicate and unsorted order\", \"actual\": \"a\", \"expected\": \"b\", \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":38.238,"exit_code":1,"observations":[{"actual":null,"check":"empty collection","expected":null,"passed":true},{"actual":"a","check":"singleton collection","expected":null,"passed":false},{"actual":"b","check":"two distinct elements","expected":"a","passed":false},{"actual":"c","check":"nonadjacent duplicate and unsorted order","expected":"b","passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"empty collection\", \"actual\": null, \"expected\": null, \"passed\": true}, {\"check\": \"singleton collection\", \"actual\": \"a\", \"expected\": null, \"passed\": false}, {\"check\": \"two distinct elements\", \"actual\": \"b\", \"expected\": \"a\", \"passed\": false}, {\"check\": \"nonadjacent duplicate and unsorted order\", \"actual\": \"c\", \"expected\": \"b\", \"passed\": false}], \"passed\": false}\n"}},"member_only":{"stages":["fixed"],"fields":["implementations.fixed","verification.fixed","harness","repair"],"note":"The verified repair, its recorded checks, the repair description, and the scoring harness are available to members."}}