(define classify-triangle
  (lambda (side1 side2 side3)
    (cond
      ((or (< side1 1) (< side2 1) (< side3 1))
       (error "classify-triangle: sides must have positive length"))
      ((and (eq? side1 side2) (eq? side2 side3)) 'equilateral)
      ((or (eq? side1 side2) 
           (eq? side2 side3)
           (eq? side3 side1))
       'isoceles)
      (else 'scalene))))