I normally use C-x C-e to execute a form. Since I use either SBCL (with threading enabled) or ACL, slime creates a new thread to execute the form (rather than executing the form in the REPL thread).
Occasionally, I need to interrupt or debug the execution. I know that I can use slime-list-threads to see the list of threads. How do I determine which thread is the one that is executing my form? How do I interrupt that thread safely (so that the stack is unwound cleanly)?