;;; source: http://www.billthelizard.com/2009/11/sicp-exercise-112-pascals-triangle.html

(define (pascal row col)
   (cond ((< row col) #f)
         ((or (= 0 col) (= row col)) 1)
         (else (+ (pascal (- row 1) col)
                  (pascal (- row 1) (- col 1))))))