(define sum-of-number-tree
  (lambda (tree)
    ; If it's just a simple value, use that value
    (if (not (pair? tree))
        tree
        (let (
              ; Sum everything in the left subtree
              (left-sum (sum-of-number-tree (car tree)))
              ; Sum everything in the right subtre
              (right-sum (sum-of-number-tree (cdr tree))))
          ; Put 'em together
          (+ left-sum right-sum)))))

(define depth
  (lambda (tree)
    (if (not (pair? tree))
        0
        (let ((left-depth (depth (car tree)))
              (right-depth (depth (cdr tree))))
          (+ 1 (max left-depth right-depth))))))

(define number-tree?
  (lambda (tree)
    (if (not (pair? tree))
        (number? tree)
        (let ((left (number-tree? (car tree)))
              (right (number-tree? (cdr tree))))
          (and left right)))))