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