::: (eval exp env) returns the result of evaluating
;;; expression exp in the environment env.
(define (eval exp env)
  (cond ((self-evaluating? exp) exp)
        ((variable? exp) (lookup-variable-value exp env))
        ((quoted? exp) (text-of-quotation exp))
        ((assignment? exp) (eval-assignment exp env))
        ((definition? exp) (eval-definition exp env))
        ((if? exp) (eval-if exp env))
        ((lambda? exp)
         (make-procedure (lambda-parameters exp)
                         (lambda-body exp)
                         env))
        ((begin? exp) 
         (eval-sequence (begin-actions exp) env))
        ((cond? exp) (eval (cond->if exp) env))
        ((application? exp)
         (apply (eval (operator exp) env)
                (list-of-values (operands exp) env)))
        (else
         (error "Unknown expression type -- EVAL" exp))))

;;; (apply procedure arguments) returns the result of 
;;; applying procedure to arguments.
(define (apply procedure arguments)
  (cond ((primitive-procedure? procedure)
         (apply-primitive-procedure procedure arguments))
        ((compound-procedure? procedure)
         (eval-sequence
           (procedure-body procedure)
           (extend-environment
             (procedure-parameters procedure)
             arguments
             (procedure-environment procedure))))
        (else
         (error
          "Unknown procedure type -- APPLY" procedure))))

;;; Auxiliary functions

(define (self-evaluating? exp)
  (cond ((number? exp) true)
        ((string? exp) true)
        (else false)))

;;; (lookup-variable-value var env) searches environment env 
;;; for variable var and returns the corresponding value.
(define (lookup-variable-value var env) ... )

;;; (eval-assignment exp env) sets the variable of expression exp
;;; to the value of exp in env.
(define (eval-assignment exp env)
  (set-variable-value! (assignment-variable exp)
                       (eval (assignment-value exp) env)
                       env)
  'ok)

;;; (list-of-values exps env) returns a list of the values 
;;; of the expressions in exps in env.
(define (list-of-values exps env)
  (if (no-operands? exps)
      '()
      (map (lambda (operand) (eval operand env)) exps)))

;;; (extend-environment pars args env) extends env by binding 
;;; each argument in arg to the corresponding parameter in pars.
(define (extend-environment pars args env) ... )

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