FA-4236 / Collection algebra / Open access

Stable set union · case 01

The operation returns a result or retained state that violates this contract: Return unique union values in first appearance order across left then right.

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

ROOT CAUSE

Sorting the union erases operand encounter order.

VERIFIED REPAIR

Return unique union values in first appearance order across left then right.

Unsuccessful approach: Reversing the operands gives right-side encounters priority.

Case contract

Return unique union values in first appearance order across left then right. 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 sorted(set(x+y))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('both empty', solve([], []), [])
check('empty right operand', solve(['a'], []), ['a'])
check('empty left operand', solve([], ['b']), ['b'])
check('overlap and multiplicity', solve(['b', 'a', 'b'], ['b', 'c', 'c']), ['b', 'a', 'c'])
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
both empty	`[]`	`[]`	Passed
empty right operand	`['a']`	`['a']`	Passed
empty left operand	`['b']`	`['b']`	Passed
overlap and multiplicity	`['a', 'b', 'c']`	`['b', 'a', 'c']`	Failed

SHA-256 / 072bec4d6cedb1fbf4af642ca69181657b6a67e8d249763bbaa5f40ea5e9e6e4

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(y+x))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('both empty', solve([], []), [])
check('empty right operand', solve(['a'], []), ['a'])
check('empty left operand', solve([], ['b']), ['b'])
check('overlap and multiplicity', solve(['b', 'a', 'b'], ['b', 'c', 'c']), ['b', 'a', 'c'])
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
both empty	`[]`	`[]`	Passed
empty right operand	`['a']`	`['a']`	Passed
empty left operand	`['b']`	`['b']`	Passed
overlap and multiplicity	`['b', 'c', 'a']`	`['b', 'a', 'c']`	Failed

SHA-256 / c6f0de56aa810c4a1dfc6139a59a66afa5c1c24e5fcff936f34498eb4d6b3f17

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 list(dict.fromkeys(x+y))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('both empty', solve([], []), [])
check('empty right operand', solve(['a'], []), ['a'])
check('empty left operand', solve([], ['b']), ['b'])
check('overlap and multiplicity', solve(['b', 'a', 'b'], ['b', 'c', 'c']), ['b', 'a', 'c'])
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
both empty	`[]`	`[]`	Passed
empty right operand	`['a']`	`['a']`	Passed
empty left operand	`['b']`	`['b']`	Passed
overlap and multiplicity	`['b', 'a', 'c']`	`['b', 'a', 'c']`	Passed

SHA-256 / b264d3c25dc93cfc69a1c3fb1ba73a58a952148f94ac42e7b0dda23e070e39cb

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 2026-09-29T14:37:37.144465+00:00.

Case digest / 139315f6b7c4c10d15a3e550bfc623de2f7cc5cd36e04538b9afbcfb5af96713