@staticmethod
def ee_checksum(value):
    """Takes a string of digits as input, returns check digit."""
    for i in (1, 3):
        check = 0
        for c in value:
            check += i * int(c)
            i = i % 9 + 1
        check %= 11
        if check < 10:
            return check
    return check % 10
