PETRI/TEST> (defun my-factorial (n) (if (= n 0) 1 (* n (my-factorial (1- n)))))
MY-FACTORIAL
PETRI/TEST> (trace my-factorial)
(MY-FACTORIAL)
PETRI/TEST> (my-factorial 4)
  0: (PETRI/TEST::MY-FACTORIAL 4)
    1: (PETRI/TEST::MY-FACTORIAL 3)
      2: (PETRI/TEST::MY-FACTORIAL 2)
        3: (PETRI/TEST::MY-FACTORIAL 1)
          4: (PETRI/TEST::MY-FACTORIAL 0)
          4: MY-FACTORIAL returned 1
        3: MY-FACTORIAL returned 1
      2: MY-FACTORIAL returned 2
    1: MY-FACTORIAL returned 6
  0: MY-FACTORIAL returned 24
24