FA-4136 / Sequence semantics / Open access

Drop first not all matching · case 01

The operation returns a result or retained state that violates this contract: Remove exactly the first positional element.

Verified by executionVariant 1 · 5 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Value-based filtering removes later elements equal to the first.

VERIFIED REPAIR

Remove exactly the first positional element.

Unsuccessful approach: Deduplicating the suffix removes legitimate repeated survivors.

Case contract

Remove exactly the first positional element. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.

Why this case matters

A controlled local-runtime regression for collection APIs, language semantics, or ownership wrappers. Fixtures include boundary and interaction cases.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x, y=None):
    return [v for v in x if v != x[0]] if x else []
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty collection', solve([]), [])
check('singleton collection', solve(['a']), [])
check('two distinct elements', solve(['a', 'b']), ['b'])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), ['a', 'b', 'c'])
check('duplicate suffix is retained', solve(['c','b','b']), ['b','b'])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
empty collection	`[]`	`[]`	Passed
singleton collection	`[]`	`[]`	Passed
two distinct elements	`['b']`	`['b']`	Passed
nonadjacent duplicate and unsorted order	`['a', 'c']`	`['a', 'b', 'c']`	Failed
duplicate suffix is retained	`['b', 'b']`	`['b', 'b']`	Passed

SHA-256 / deae9487337f095e1d0bd85a60c9ab3ed1d48b2c782fc80574f192e0adc7b94d

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x, y=None):
    return list(dict.fromkeys(x[1:]))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty collection', solve([]), [])
check('singleton collection', solve(['a']), [])
check('two distinct elements', solve(['a', 'b']), ['b'])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), ['a', 'b', 'c'])
check('duplicate suffix is retained', solve(['c','b','b']), ['b','b'])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
empty collection	`[]`	`[]`	Passed
singleton collection	`[]`	`[]`	Passed
two distinct elements	`['b']`	`['b']`	Passed
nonadjacent duplicate and unsorted order	`['a', 'b', 'c']`	`['a', 'b', 'c']`	Passed
duplicate suffix is retained	`['b']`	`['b', 'b']`	Failed

SHA-256 / f4b466ec401b54953d3a93b93e0ca937fd3edd455b195e766397f11a2380280e

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x, y=None):
    return x[1:]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('empty collection', solve([]), [])
check('singleton collection', solve(['a']), [])
check('two distinct elements', solve(['a', 'b']), ['b'])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), ['a', 'b', 'c'])
check('duplicate suffix is retained', solve(['c','b','b']), ['b','b'])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
empty collection	`[]`	`[]`	Passed
singleton collection	`[]`	`[]`	Passed
two distinct elements	`['b']`	`['b']`	Passed
nonadjacent duplicate and unsorted order	`['a', 'b', 'c']`	`['a', 'b', 'c']`	Passed
duplicate suffix is retained	`['b', 'b']`	`['b', 'b']`	Passed

SHA-256 / 83cae5255b9300c7debb5f0eaf6ae7143c2c89d36433f0b726f7642aa69c7907

Verification & scope

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.

Observations recorded using Python 3.12.14 at 2026-09-29T14:37:36.172257+00:00.

Case digest / a30bd3a8308c113d2f134c874d16a88513a53c3794315563a72d1aa6724eef91