A better version of sublis reuses parts of the original tree when possible:

(define (sublis alist tree)
  (if (pair? tree)
      (let ((left (sublis alist (car tree)))
            (right (sublis alist (cdr tree))))
        (if (and (eq? left (car tree))
                 (eq? right (cdr tree)))
            tree
            (cons left right)))
      (let ((new (assv tree alist)))
        (if new
            (cdr new)
            tree) ) ) )

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