(define (tfr t) (if (pair? t) (let s ((t t)) (if (pair? (car t))
  (s (cons (caar t) (cons (cdar t) (cdr t)))) t)) (cons t #f)))
(define (sf x y) (let ((x (tfr x)) (y (tfr y))) (and (eq? (car x) (car y))
     (or (not (or (cdr x) (cdr y))) (sf (cdr x) (cdr y))))))

; 37 paren pairs