FA-4211 / Sequence semantics / Open access

First occurrence positions · case 01

The operation returns a result or retained state that violates this contract: Return positions at which each distinct value first appears.

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

ROOT CAUSE

Last occurrences are selected instead of first occurrences.

VERIFIED REPAIR

Return positions at which each distinct value first appears.

Unsuccessful approach: Every position is retained even when a value already appeared.

Case contract

Return positions at which each distinct value first appears. 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 [i for i,v in enumerate(x) if v not in x[i+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']), [0])
check('two distinct elements', solve(['a', 'b']), [0, 1])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), [0, 1, 3])
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	`[0]`	`[0]`	Passed
two distinct elements	`[0, 1]`	`[0, 1]`	Passed
nonadjacent duplicate and unsorted order	`[1, 2, 3]`	`[0, 1, 3]`	Failed

SHA-256 / e299f820571d7467695bf38d88cfa190eacf3ba55da7f0a4ed2d77d51d357e80

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(range(len(x)))
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']), [0])
check('two distinct elements', solve(['a', 'b']), [0, 1])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), [0, 1, 3])
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	`[0]`	`[0]`	Passed
two distinct elements	`[0, 1]`	`[0, 1]`	Passed
nonadjacent duplicate and unsorted order	`[0, 1, 2, 3]`	`[0, 1, 3]`	Failed

SHA-256 / 4fcd2d2c63647f6a1868a4cfc0426d8cdbaedd15479d44f9deeb4ae75edb3173

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 [i for i,v in enumerate(x) if v not in x[:i]]
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']), [0])
check('two distinct elements', solve(['a', 'b']), [0, 1])
check('nonadjacent duplicate and unsorted order', solve(['b', 'a', 'b', 'c']), [0, 1, 3])
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	`[0]`	`[0]`	Passed
two distinct elements	`[0, 1]`	`[0, 1]`	Passed
nonadjacent duplicate and unsorted order	`[0, 1, 3]`	`[0, 1, 3]`	Passed

SHA-256 / af401c2d3e6f49fa10a87bc5b61a2d074439321800d35b6f0beb7683d71b7ba1

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 2026-09-29T14:37:36.944394+00:00.

Case digest / bdaa35be08ffd13fcb205c25a05d363e0db67e167986cdfe7dc757c43e0589cc