FA-4281 / Collection algebra / Open access

Alternating interleave preserves tail · case 01

The operation returns a result or retained state that violates this contract: Alternate left and right elements and retain the longer trailing suffix.

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

ROOT CAUSE

Zip truncation discards the longer trailing suffix.

VERIFIED REPAIR

Alternate left and right elements and retain the longer trailing suffix.

Unsuccessful approach: Concatenation fails to alternate between operands.

Case contract

Alternate left and right elements and retain the longer trailing suffix. 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 pair in zip(x,y) for v in pair]
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', 'b', 'a', 'c', 'b', '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']`	Failed
empty left operand	`[]`	`['b']`	Failed
overlap and multiplicity	`['b', 'b', 'a', 'c', 'b', 'c']`	`['b', 'b', 'a', 'c', 'b', 'c']`	Passed

SHA-256 / a54042707798e1c2f2d473a461537974aa5b3e577f5274586924e8891afebd74

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+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', 'b', 'a', 'c', 'b', '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', 'b', 'b', 'c', 'c']`	`['b', 'b', 'a', 'c', 'b', 'c']`	Failed

SHA-256 / b8be59ba9ee1518f450683d0529cb8e42a6cf8007b4d18b90caff8c039d303cd

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 [v for i in range(max(len(x),len(y))) for v in (x[i:i+1]+y[i:i+1])]
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', 'b', 'a', 'c', 'b', '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', 'b', 'a', 'c', 'b', 'c']`	`['b', 'b', 'a', 'c', 'b', 'c']`	Passed

SHA-256 / 0168001d77ad77e59ab1322275bcdae76aa9b36d1c27b2b2de84d00a69db634b

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 2026-09-29T14:37:37.541311+00:00.

Case digest / ada5a1f2df5aec717e6ac3911e6099ef790484efd2045ad8966ca6a91865993f