(define xml '(*TOP* (*PI* (xml "version=\"1.0\"") (foo (@ (bar "baz")) (qux "quux")))))

(define (xml-identity xml)
  (pre-post-order*
   xml
   `((*default* . ,(lambda x x))
     (*text* . ,(lambda (_ text) text)))))

(xml-identity xml)
;; (*TOP* ((*PI* ((xml ("version=\"1.0\"")) (foo ((@ ((bar ("baz")))) (qux ("quux"))))))))