(define look-up-value ; from chapter 8 (lambda (name) (cond ((equal? name '+) +) ((equal? name '*) *) ((equal? name '-) -) ((equal? name '/) /) (else (error "Unrecognized name" name))))) (define operator? (lambda (token) (member token '(+ - * /)))) (define lower-precedence? (lambda (op-1 op-2) (and (member op-1 '(+ -)) (member op-2 '(* /)))))The tokenize procedure is described fully in the sidebar on pp. 433-34.