Sorry, totally wrong, it should be:

'ab' = (1*(26**1)) + (2*(26**0))

'ab' -- = (1*(26**1)) + ((2*(26**0)) - (1*(26*0)))

        = (1*(26**1)) + (1*(26**0))

        = 'aa'
[download]

But ideally a string containing only alphanumerics with an alphabet in upper and lower case could be handled as a Base 62 number which would also allow decrement, because the question with the above is how does it handle numerics and upper case. Base 62 would solve that.