Concurrency, interactivity, mutability, choose two

You have a Common Lisp program running, multiple threads listening for network connections, others processing data in the background. You were really careful making sure that there are no data access conflicts. Concurrency, check. You use Emacs and SLIME to connect to your server, inspect its state. Interactivity, check. You realize a global hash table contains an incorrect value; no problem, just…