(define classify-triangle
  (lambda (side1 side2 side3)
    (cond
      ((or (<= side1 0) (<= side2 0) (<= side3 0))
       (error "classify-triangle: sides must have positive length"))
      ((and (= side1 side2) (= side2 side3)) 'equilateral)
      ((= side1 side2) 'isoceles)
      (else 'scalene))))