The "Computer Software" version of this paper has a copy-and-paste
error in Scheme code in the Appendix.

  Wrong:

    (define (dynamic-operation? x)
      (and (pair? x)
  	   (dynamic-symbol? x)))

  Correct:

    (define (dynamic-operation? x)
      (and (pair? x)
  	   (symbol? (car x))
  	   (char=? #\_ (string-ref (symbol->string (car x)) 0))))

We thank Mr. Kouki Nishizawa for pointing out this error.