def __check_natural(items, minimum):
    """Checks a list of strings whether items are valid natural numbers.

    Args:
        items (list of [str,]): The list of items to test.
        minimum (int): The minimum acceptable natural number (0 or 1).

    Returns:
        list of [int,]: The list of items converted to natural numbers. Returns
            None if any one test fails.
    """
    for i, item in enumerate(items):
        if '.' in item:
            print('Failed on item: "{}"'.format(item))
            return None
        try:
            items[i] = int(item)
        except ValueError:
            print('Failed on item: "{}"'.format(item))
            return None
        if items[i] < minimum:
            print('Failed on item: "{}"'.format(item))
            return None
    return items
