sol-2.31.tex

The first answer to exercise~\ref[2.30] gets abstraced into
\begtt\scm
(define (tree-map proc tree)
  (cond ((null? tree) nil)
        ((not (pair? tree)) (proc tree))
        (else (cons (tree-map proc (car tree))
                    (tree-map proc (cdr tree))))))
\endtt

While the second one becomes
\begtt\scm
(define (tree-map proc tree)
  (map (lambda (sub-tree)
         (if (pair? sub-tree)
             (tree-map proc sub-tree)
             (proc sub-tree)))
       tree))
\endtt