<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def luhn_checksum(num: str) -> str: """Calculate a checksum for num using the Luhn algorithm. :param num: The number to calculate a checksum for as a string. :return: Checksum for number. """

    check = 0
    for i, s in enumerate(reversed(num)):
        sx = int(s)
        sx = sx * 2 if i % 2 == 0 else sx
        sx = sx - 9 if sx > 9 else sx
        check += sx
    return str(check * 9 % 10)