This exercise asks you to write a procedure which returns a
procedure. However, an unnamed procedure created with lambda
is difficult to invoke it recursively as the procedure has no name.
This problem can be solved as follows.
(define (iterative-improve .......)
(define (iter .....) ........)
iter) <------------------------- return this function
This exercise also asks you to rewrite sqrt and fixed-point with
iterative-improve just like:
(define (sqrt x)
.........
((iterative-improve .....) 1.0)) <---- 1.0 is the first guess
(define (fixed-point f g)
......
((iterative-improve ....) g))