{"abstract":"The operation returns a result or retained state that violates this contract: Alternate left and right elements and retain the longer trailing suffix.","category":"Collection algebra","checks":4,"contract":"Alternate left and right elements and retain the longer trailing suffix. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","contract_signature":"x, y=None","evaluation_group":"model-68baeaecfcee13e9","failed_approach":"Concatenation fails to alternate between operands.","family":"xr-alternating-interleave-preserves-tail","id":"FA-4281","implementations":{"attempt":{"sha256":"b8be59ba9ee1518f450683d0529cb8e42a6cf8007b4d18b90caff8c039d303cd","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return 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', 'b', 'a', 'c', 'b', '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":"a54042707798e1c2f2d473a461537974aa5b3e577f5274586924e8891afebd74","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return [v for pair in zip(x,y) for v in pair]\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', 'b', 'a', 'c', 'b', '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-alternating-interleave-preserves-tail","generated_at":"2026-09-29T14:37:37.541311+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":"Zip truncation discards the longer trailing suffix.","sha256":"504cc9cc81a87d1dcd618d290328c437758d8cd990814616db3907a5ad987a6f","title":"Alternating interleave preserves tail · 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":39.57,"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","b","b","c","c"],"check":"overlap and multiplicity","expected":["b","b","a","c","b","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\", \"b\", \"b\", \"c\", \"c\"], \"expected\": [\"b\", \"b\", \"a\", \"c\", \"b\", \"c\"], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":38.898,"exit_code":1,"observations":[{"actual":[],"check":"both empty","expected":[],"passed":true},{"actual":[],"check":"empty right operand","expected":["a"],"passed":false},{"actual":[],"check":"empty left operand","expected":["b"],"passed":false},{"actual":["b","b","a","c","b","c"],"check":"overlap and multiplicity","expected":["b","b","a","c","b","c"],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"both empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"empty right operand\", \"actual\": [], \"expected\": [\"a\"], \"passed\": false}, {\"check\": \"empty left operand\", \"actual\": [], \"expected\": [\"b\"], \"passed\": false}, {\"check\": \"overlap and multiplicity\", \"actual\": [\"b\", \"b\", \"a\", \"c\", \"b\", \"c\"], \"expected\": [\"b\", \"b\", \"a\", \"c\", \"b\", \"c\"], \"passed\": true}], \"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."}}