How can I write a loop using letters instead of numbers so it looks like this:

2 sets used 'a-z' and 'A-Z'

aaaaa

aaaaA

aaaAa

aaAaa

and so on:

aaaAA

then on to b:

bbbbb

bbbbA

I can do it with numbers, something like this:

```
for i in range(10):
for j in range(10):
for k in range(10):
print i,j,k
```

How does this work with letters?