Exercise 5.8.  The following register-machine code is ambiguous, because 
the label here is defined more than once:

start
  (goto (label here))
here
  (assign a (const 3))
  (goto (label there))
here
  (assign a (const 4))
  (goto (label there))
there

With the simulator as written, what will the contents of register a be 
when control reaches there? Modify the extract-labels procedure so that 
the assembler will signal an error if the same label name is used to 
indicate two different locations.

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(define (extract-labels text receive)
  (if (null? text)
      (receive '() '())
      (extract-labels (cdr text)
       (lambda (insts labels)
         (let ((next-inst (car text)))
           (if (symbol? next-inst)
               (if (assoc next-inst labels)                      ;;; modified
                   (error "Label redefinition" next-inst)
                   (receive insts
                            (cons (make-label-entry next-inst
                                                    insts)
                                  labels)))
               (receive (cons (make-instruction next-inst)
                              insts)
                        labels)))))))