* (foo 42)
; in: FOO 42
;     (FOO 42)
; 
; caught STYLE-WARNING:
;   undefined function: COMMON-LISP-USER::FOO
; 
; compilation unit finished
;   Undefined function:
;     FOO
;   caught 1 STYLE-WARNING condition

debugger invoked on a UNDEFINED-FUNCTION in thread
#<THREAD "main thread" RUNNING {1004A88103}>:
  The function COMMON-LISP-USER::FOO is undefined.

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [CONTINUE      ] Retry calling FOO.
  1: [USE-VALUE     ] Call specified function.
  2: [RETURN-VALUE  ] Return specified values.
  3: [RETURN-NOTHING] Return zero values.
  4: [ABORT         ] Exit debugger, returning to top level.

("undefined function" 42)

0] (defun foo (x) (* x x))
; No debug variables for current frame: using EVAL instead of EVAL-IN-FRAME.
FOO

0] 0
1764