FA-4146 / Sequence semantics / Open access

Interior excludes both ends · case 01

The operation returns a result or retained state that violates this contract: Return the positional interior, excluding first and last elements.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The trailing endpoint remains in the selected interior.

VERIFIED REPAIR

Return the positional interior, excluding first and last elements.

Unsuccessful approach: Removing only the final endpoint leaves the leading endpoint.

Case contract

Return the positional interior, excluding first and last elements. 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 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']), [])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), ['a', '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']`	`[]`	Failed
nonadjacent duplicate and unsorted order	`['a', 'b', 'c']`	`['a', 'b']`	Failed

SHA-256 / 49add9dbe2997c910ff811d2e6331de12dffe0cf7adea5addf6c4af53a5bd8b0

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 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']), [])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), ['a', '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	`['a']`	`[]`	Failed
nonadjacent duplicate and unsorted order	`['b', 'a', 'b']`	`['a', 'b']`	Failed

SHA-256 / 6284a31bb379c75ebb1cf3c3038a6d38a604d6e7a37f50f140fe62eeded867b8

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:-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']), [])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), ['a', '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	`[]`	`[]`	Passed
nonadjacent duplicate and unsorted order	`['a', 'b']`	`['a', 'b']`	Passed

SHA-256 / 7fa550bc2ed4620ef11de2bc262f380e66863d1c4dac3451d145840fafb275f8

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 2026-09-29T14:37:36.192434+00:00.

Case digest / 2f9eef75eb99c1ae2dfc5346d51560323bd19f431056acf665125790fd41fc41