(f1 5)                              |  f1(5)
 (* 5 (f1 4))                        |  (5 * f1(4))
 (* 5 (* 4 (f1 3)))                  |  (5 * (4 * f1(3)))
 (* 5 (* 4 (* 3 (f1 2))))            |  (5 * (4 * (3 * f1(2))))
 (* 5 (* 4 (* 3 (* 2 (f1 1)))))      |  (5 * (4 * (3 * (2 * f1(1)))))
 (* 5 (* 4 (* 3 (* 2 (* 1 (f1 0))))))|  (5 * (4 * (3 * (2 * (1 * f1(0))))))
 (* 5 (* 4 (* 3 (* 2 (* 1 1)))))     |  (5 * (4 * (3 * (2 * (1 * (1))))))
 (* 5 (* 4 (* 3 (* 2 1))))           |  (5 * (4 * (3 * (2 * (1)))))
 (* 5 (* 4 (* 3 2)))                 |  (5 * (4 * (3 * (2))))
 (* 5 (* 4 6))                       |  (5 * (4 * (6)))
 (* 5 24)                            |  (5 * (24))
 120                                 |  (120)
         (SCHEME/LISP)               |           (C/C++)