converting numbers to strings in arbitrary base (up to 36)

[Mirko]

Hi,

This is an elisp question:

I can specify integers in base 2-36 using #xyr..., and I can read them from a 
file.

But is there a way to write an integer in arbitrary base (again, 2-36) to a 
file?

I looked at string-to-number, format, and calc

In greater detail,

I have a file where I want to keep a counter (in base 36).
When needed, I want to open the file, read the number.
Occasionally, I want to increment the number, and write it back out.

I can handle reading and writing :-).  It is writing the base 36 that
I don't know how to handle.

Thank you,

Mirko