In order to get to the arc prompt, you'll need to run the application server (asv) in a separate thread, you can do so with the following code:
(= app (thread (asv)))
and, you can stop the server with the this code:
(break-thread app)
Once Arc's app server is running in a separate thread you'll have full access to the Arc repl and you'll then be able to modify your application on the fly.
