p54.ss

;; P54A (*) Check whether a given term represents a binary tree
;; Write a predicate istree which returns true if and only if its argument is a list representing a binary tree.
;; Example:
;; * (istree (a (b nil nil) nil))
;; T
;; * (istree (a (b nil nil)))
;; NIL

(define (istree? lst)
  (or (null? lst)
      (and (= (length lst) 3)
           (istree? (cadr lst))
           (istree? (caddr lst)))))