FA-4336 / Container mutation / Open access

List remove first equal only · case 01

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.

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

ROOT CAUSE

Filtering by inequality removes every equal value rather than one occurrence.

THE FAILURE

Filtering by inequality removes every equal value rather than one occurrence.

Unsuccessful approach: Removing the last equal value changes the surviving positional order.

Case 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.

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 != y]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('separated duplicates', solve(['a', 'b', 'a'], 'a'), ['b', 'a'])
check('missing', solve(['a'], 'z'), ['a'])
check('empty', solve([], 'z'), [])
check('only match', solve(['a'], 'a'), [])
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
separated duplicates	`['b']`	`['b', 'a']`	Failed
missing	`['a']`	`['a']`	Passed
empty	`[]`	`[]`	Passed
only match	`[]`	`[]`	Passed

SHA-256 / 197432a8948ad59540a45826c89ffa59e1e71877912a0187b1d3d7d53b4da393

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(x, y=None):
    a=list(x)
    if y in a: a.pop(len(a)-1-a[::-1].index(y))
    return a
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('separated duplicates', solve(['a', 'b', 'a'], 'a'), ['b', 'a'])
check('missing', solve(['a'], 'z'), ['a'])
check('empty', solve([], 'z'), [])
check('only match', solve(['a'], 'a'), [])
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
separated duplicates	`['a', 'b']`	`['b', 'a']`	Failed
missing	`['a']`	`['a']`	Passed
empty	`[]`	`[]`	Passed
only match	`[]`	`[]`	Passed

SHA-256 / dc6ba11c87abe6dd1c92217bc02e2aaeee0fab2051b8e98a4d3cf78020757e2e

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 4 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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 2026-09-29T14:37:38.066045+00:00.

Case digest / 2ab1a549f52cca5185aebe10ae623e8f417812b8dc50b81f14099bbcea8b7443