;; substitution.scm
;;
;; an example of our previous "substitution model" of evalution
;;
;; 

(define (f x) (2 * x))

(define (g y)
  (let ((z (* 2 y))
        (+ (f z)
           (f y)))))

(g 10)

;; -- Here's what the substitution model looks like ...
;;    plugging in by hand.
;;
;;   (g 10)
;;
;;   (let ((z (* 2 10))      ;; substitute definition of g with y => 10
;;         (+ (f z)
;;            (f 10)))))
;;
;;   (+ (f 20)               ;; substitute into (let ..) value of z
;;      (f 10))       
;;
;;   (+ (* 2 20)             ;; subsitute definition of f with x => 20 or 10
;;      (* 2 10))

;;
;;