Calculate and return control digit for given list of digits based on
    ISO7064, MOD 11,10 standard.

def checksum(digits):
    """
    Calculate and return control digit for given list of digits based on
    ISO7064, MOD 11,10 standard.
    """
    remainder = 10
    for digit in digits:
        remainder = (remainder + digit) % 10
        if remainder == 0:
            remainder = 10
        remainder = (remainder * 2) % 11

    control_digit = 11 - remainder
    if control_digit == 10:
        control_digit = 0
    return control_digit