(define classify-triangle
  (lambda (side1 side2 side3)
    (cond
      ((< (+ side1 side2) side3) 
       (error "classify-triangle: sides do not describe a triangle"))
      ((and (= side1 side2) (= side2 side3)) 'equilateral)
      ((or (= side1 side2) 
           (= side2 side3)
           (= side3 side1))
       'isoceles)
      (else 'scalene))))