For the second part: with restarts and conditions, CL pulls up an interactive debugger that asks if you would like to abort, continue/retry calling undefined or

For the second part: with restarts and conditions, CL pulls up an interactive debugger that asks if you would like to abort, continue/retry calling undefined or supply a definition now, amongst others. Evalutating this in CL (SBCL) (undef-fn) gives: Restarts: 0: [CONTINUE] Retry calling UNDEF-FN. 1: [USE-VALUE] Call specified function. 2: [RETURN-VALUE] Return specified values. 3: [RETURN-NOTHING] Return zero values. 4: [RETRY] Retry SLY interactive evaluation request. 5: [\*ABORT] Return to SLY's top level. 6: [ABORT] abort thread (#<THREAD "slynk-worker" RUNNING {100288C7E3}>) [... as well as a clipped stack trace ...] Pressing 1 will allow you to name an alternative function in a minibuffer prompt, 2 lets you set the return value of the undefined fn, etc.