CL-USER> (defun factorial (n)
           (if (= n 1)
               1
               (* n (factorial (- n 1)))))
FACTORIAL
CL-USER> (trace factorial)
(FACTORIAL)
CL-USER> (factorial 6)
  0: (FACTORIAL 6)
    1: (FACTORIAL 5)
      2: (FACTORIAL 4)
        3: (FACTORIAL 3)
          4: (FACTORIAL 2)
            5: (FACTORIAL 1)
            5: FACTORIAL returned 1
          4: FACTORIAL returned 2
        3: FACTORIAL returned 6
      2: FACTORIAL returned 24
    1: FACTORIAL returned 120
  0: FACTORIAL returned 720
720