interating letters

[Holland King]

how do you interate a letter or string? for instance:
a + 1 = b
or 
aa + 1 = ab
etc. in c it is similar to above and i am guessing there is an equally 
simple way to do it in python, i just can't figure it out. thank you
for your time and help.

-- 
---
Joseph Holland King  | "God whispers to us in our pleasures, speaks in our
                     |  conscience, but shouts in our pains: it is His
                     |  megaphone to rouse a deaf world." C. S. Lewis