{"abstract":"The operation returns a result or retained state that violates this contract: Remove the first equal value, leaving later equal occurrences intact; missing value is a no-op.","category":"Container mutation","checks":4,"contract":"Remove the first equal value, leaving later equal occurrences intact; missing value is a no-op. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.","contract_signature":"x, y=None","evaluation_group":"model-c441d9248885a16f","failed_approach":"Removing the last equal value changes the surviving positional order.","family":"xr-list-remove-first-equal-only","id":"FA-4336","implementations":{"attempt":{"sha256":"dc6ba11c87abe6dd1c92217bc02e2aaeee0fab2051b8e98a4d3cf78020757e2e","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(x, y=None):\n    a=list(x)\n    if y in a: a.pop(len(a)-1-a[::-1].index(y))\n    return a\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('separated duplicates', solve(['a', 'b', 'a'], 'a'), ['b', 'a'])\ncheck('missing', solve(['a'], 'z'), ['a'])\ncheck('empty', solve([], 'z'), [])\ncheck('only match', solve(['a'], 'a'), [])\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":"197432a8948ad59540a45826c89ffa59e1e71877912a0187b1d3d7d53b4da393","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 x if v != y]\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\ncheck('separated duplicates', solve(['a', 'b', 'a'], 'a'), ['b', 'a'])\ncheck('missing', solve(['a'], 'z'), ['a'])\ncheck('empty', solve([], 'z'), [])\ncheck('only match', solve(['a'], 'a'), [])\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-list-remove-first-equal-only","generated_at":"2026-09-29T14:37:38.066045+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":"Filtering by inequality removes every equal value rather than one occurrence.","sha256":"2ab1a549f52cca5185aebe10ae623e8f417812b8dc50b81f14099bbcea8b7443","title":"List remove first equal only · 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":37.123,"exit_code":1,"observations":[{"actual":["a","b"],"check":"separated duplicates","expected":["b","a"],"passed":false},{"actual":["a"],"check":"missing","expected":["a"],"passed":true},{"actual":[],"check":"empty","expected":[],"passed":true},{"actual":[],"check":"only match","expected":[],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"separated duplicates\", \"actual\": [\"a\", \"b\"], \"expected\": [\"b\", \"a\"], \"passed\": false}, {\"check\": \"missing\", \"actual\": [\"a\"], \"expected\": [\"a\"], \"passed\": true}, {\"check\": \"empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"only match\", \"actual\": [], \"expected\": [], \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":39.096,"exit_code":1,"observations":[{"actual":["b"],"check":"separated duplicates","expected":["b","a"],"passed":false},{"actual":["a"],"check":"missing","expected":["a"],"passed":true},{"actual":[],"check":"empty","expected":[],"passed":true},{"actual":[],"check":"only match","expected":[],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"separated duplicates\", \"actual\": [\"b\"], \"expected\": [\"b\", \"a\"], \"passed\": false}, {\"check\": \"missing\", \"actual\": [\"a\"], \"expected\": [\"a\"], \"passed\": true}, {\"check\": \"empty\", \"actual\": [], \"expected\": [], \"passed\": true}, {\"check\": \"only match\", \"actual\": [], \"expected\": [], \"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."}}