dataset/problem_8/buggy.py

def flatten_one_level(items: list) -> list:
    """Flatten only one nesting level."""
    result = []
    for item in items:
        # BUG: recursively flattens all levels instead of one level.
        if isinstance(item, list):
            result.extend(flatten_one_level(item))
        else:
            result.append(item)
    return result