def genMod11Cipher(cprNumbStr):
    """Generate the 10th control cipher given the 9 previous. Returns string 
    holding all 10 ciphers. Return None if control cipher cannot be generated.
    
    Input:
        cprNumbStr, str[9], holds 9 first ciphers of CPR number
    Output: 
        fullCprNumber, None or str[10], full CPR number at succes, None at 
            failure.
    """
    
    sumation = 0
    cntrlVect = [4,3,2,7,6,5,4,3,2,1]
    for index in range(9): #Notice that the loop is only over 9 indices.
        sumation += int(cprNumbStr[index]) * cntrlVect[index]
    rem = sumation % 11
    if rem == 0:
        cipher10 = 0
    else:
        cipher10 = 11-rem #Calc. how much should be added to rem to make the remainder zero
    
    if cipher10 != 10: #Cipher most be integer btw 0-9
        return cprNumbStr+str(cipher10)
    else:
        return None