1.9|

$ cd ~

SICP Exercise 1.9

The first procedure is recursive and expands as:

 (+ 4 5) 
 (inc (+ (dec 4) 5)) 
 (inc (+ 3 5)) 
 (inc (inc (+ (dec 3) 5))) 
 (inc (inc (+ 2 5))) 
 (inc (inc (inc (+ (dec 2) 5)))) 
 (inc (inc (inc (+ 1 5)))) 
 (inc (inc (inc (inc (+ (dec 1) 5))))) 
 (inc (inc (inc (inc (+ 0 5))))) 
 (inc (inc (inc (inc 5)))) 
 (inc (inc (inc 6))) 
 (inc (inc 7)) 
 (inc 8) 
  
 9 

The second is iterative and expands like so:

 (+ 4 5) 
 (+ (dec 4) (inc 5)) 
 (+ 3 6) 
 (+ (dec 3) (inc 6)) 
 (+ 2 7) 
 (+ (dec 2) (inc 7)) 
 (+ 1 8) 
 (+ (dec 1) (inc 8)) 
 (+ 0 9) 
  
 9