def control_digit(ID):
    """ compute the check digit in an Israeli ID number,
        given as a string of 8 digits
    """

    assert isinstance(ID, str)
    assert len(ID) == 8
    
    total = 0              
    for i in range(8):
        val = int(ID[i]) # converts char to int
        if i%2 == 0:     # even index (0,2,4,6)
            total += val
        else:            # odd index (1,3,5,7)
            if val < 5:
                total += 2*val  
            else:
                total += ((2*val)%10) + 1 # sum of digits in 2*val
                                          # 'tens' digit must be 1

    total = total%10             # 'ones' (rightmost) digit
    check_digit = (10-total)%10  # the complement modulo 10 of total
                                 # for example 42->8, 30->0

    return str(check_digit)