Exercise 5.9.  The treatment of machine operations above permits them to 
operate on labels as well as on constants and the contents of registers. 
Modify the expression-processing procedures to enforce the condition 
that operations can be used only with registers and constants.

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(define (make-operation-exp exp machine labels operations)
  (let ((op (lookup-prim (operation-exp-op exp) operations))
        (aprocs
         (map (lambda (e)
                (if (label-exp? e)
                    (error "Machine op applied to label")
                    (make-primitive-exp e machine labels)))
              (operation-exp-operands exp))))
    (lambda ()
      (apply op (map (lambda (p) (p)) aprocs)))))