Exercise 1.4. Observe that our model of evaluation allows for combinations whose operators are compound expressions. Use this observation to describe the behavior of the following procedure:

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

This procedure applies an operator to a and b. That operator is the compound expression (if (> b 0) + -). If b is greater than 0, the operator evaluates to +. If not, it evaluates to -.

The overall effect is to return a plus the absolute value of b.