(setf factorial (FUNCALL Y … (factorial 5) 120 The author should at least have tested a counter-example (setf factorial (red hot chilly nonsense)) I expect

(setf factorial (FUNCALL Y … (factorial 5) 120 The author should at least have tested a counter-example (setf factorial (red hot chilly nonsense)) I expect the 'evaluation' (factorial 5) 120 to be 'correct' regardless of the function definition!