{"abstract":"The operation returns a result or retained state that violates this contract: Return values whose total occurrence count is exactly one, preserving input order.","category":"Sequence semantics","checks":4,"contract":"Return values whose total occurrence count is exactly one, preserving input order. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","contract_signature":"x, y=None","evaluation_group":"model-92ba48f91547fbe9","failed_approach":"Keeping first occurrences still retains duplicated members.","family":"xr-only-singleton-members","id":"FA-4111","implementations":{"attempt":{"sha256":"07585470be31d2ffa42a7a3d46ccbc8108ad4fa8377c819faea09eaa78e407a1","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    return [v for i,v in enumerate(x) if v not in x[:i]]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('empty collection', solve([]), [])\ncheck('singleton collection', solve(['a']), ['a'])\ncheck('two distinct elements', solve(['a', 'b']), ['a', 'b'])\ncheck('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', '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":"c1527362499fd4166e8476670c3d7b0ce56e2324224d1ccbd1c583a16b17f2bd","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))\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('empty collection', solve([]), [])\ncheck('singleton collection', solve(['a']), ['a'])\ncheck('two distinct elements', solve(['a', 'b']), ['a', 'b'])\ncheck('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', '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-only-singleton-members","generated_at":"2026-09-29T14:37:35.745394+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":"Deduplication retains one copy of values that should be entirely excluded.","sha256":"54422e166504f6fb55c1f6094eb786160fb34ac6a09567cacc8707266a0898ae","title":"Only singleton members · 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":125.739,"exit_code":1,"observations":[{"actual":[],"check":"empty collection","expected":[],"passed":true},{"actual":["a"],"check":"singleton collection","expected":["a"],"passed":true},{"actual":["a","b"],"check":"two distinct elements","expected":["a","b"],"passed":true},{"actual":["b","a","c"],"check":"nonadjacent duplicate and unsorted order","expected":["a","c"],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"empty collection\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"singleton collection\", \"actual\": [\"a\"], \"expected\": [\"a\"], \"passed\": true}, {\"check\": \"two distinct elements\", \"actual\": [\"a\", \"b\"], \"expected\": [\"a\", \"b\"], \"passed\": true}, {\"check\": \"nonadjacent duplicate and unsorted order\", \"actual\": [\"b\", \"a\", \"c\"], \"expected\": [\"a\", \"c\"], \"passed\": false}], \"passed\": false}\n"},"broken":{"elapsed_ms":42.256,"exit_code":1,"observations":[{"actual":[],"check":"empty collection","expected":[],"passed":true},{"actual":["a"],"check":"singleton collection","expected":["a"],"passed":true},{"actual":["a","b"],"check":"two distinct elements","expected":["a","b"],"passed":true},{"actual":["b","a","c"],"check":"nonadjacent duplicate and unsorted order","expected":["a","c"],"passed":false}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"empty collection\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"singleton collection\", \"actual\": [\"a\"], \"expected\": [\"a\"], \"passed\": true}, {\"check\": \"two distinct elements\", \"actual\": [\"a\", \"b\"], \"expected\": [\"a\", \"b\"], \"passed\": true}, {\"check\": \"nonadjacent duplicate and unsorted order\", \"actual\": [\"b\", \"a\", \"c\"], \"expected\": [\"a\", \"c\"], \"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."}}