I'm trying understand how to interpret the output of, and use, the Lisp debugger.

I've got a pretty simple Backtrace for the evaluation of my function, but I cann't seem to work out how to use it to find out in which Lisp 'form' in my function the exception occurred.

I'd appreciate any clues as to what I should be doing, to find the source of the error.

I've attached a screen shot (if it's too small to read I can re-post it in parts), with the debug output, the function and the repl (please ignore my very wrong function, I'm just interested in learning how to use the debugger properly). In addition, I hit 'v' on the first frame to go to the source, but this resulted in the error at the bottom of the screen.