From: wooks wooks (wookiz at hotmail.com) Date: Tue Jun 20 14:50:20 EDT 2006 

http://www.htdp.org/20030926/Book/curriculumZH15.html#node_sec_11.5 I can only solve this problem by using + for the recursion. because my add function only works when at least one of the inputs is a natural number  i.e (define (add m n) (cond [(zero? n) m] [else (add1 (add m (sub1 n)))] ) ) which will obvious not work for the required recursion (add pi (multiplybypi ( n 1))).
Posted on the users mailing list. 
