Exercise 1.4

Observe that our model of evaluation allows for combinations whose operators are compound expressions. Use this observation to describe the behaviour of the following procedure:

(define (a-plus-abs-b a b)
  ((if (> b 0) + -) a b))

Adding a positive number and subtracting a negative number has the same effect as adding the absolute value of a number. We are able to change the procedure as observed. Thus + is returned as the procedure if b is greater than 0, and - otherwise.