foo.xml

<?xml version="1.0" encoding="utf-8"?>
<d:data xmlns:d="defiant-namespace" d:mi="23">
    <store d:mi="22">
        <book price="12.99" d:price="Number" d:mi="4">
            <title d:constr="String" d:mi="1">Sword of Honour</title>
            <category d:constr="String" d:mi="2">fiction</category>
            <author d:constr="String" d:mi="3">Evelyn Waugh</author>
        </book>
        <book price="8.99" d:price="Number" d:mi="9">
            <title d:constr="String" d:mi="5">Moby Dick</title>
            <category d:constr="String" d:mi="6">fiction</category>
            <author d:constr="String" d:mi="7">Herman Melville</author>
            <isbn d:constr="String" d:mi="8">0-553-21311-3</isbn>
        </book>
        <book price="8.95" d:price="Number" d:mi="13">
            <title d:constr="String" d:mi="10">50</title>
            <category d:constr="String" d:mi="11">reference</category>
            <author d:constr="String" d:mi="12">Nigel Rees</author>
        </book>
        <book price="22.99" d:price="Number" d:mi="18">
            <title d:constr="String" d:mi="14">The Lord of the Rings</title>
            <category d:constr="String" d:mi="15">fiction</category>
            <author d:constr="String" d:mi="16">J. R. R. Tolkien</author>
            <isbn d:constr="String" d:mi="17">0-395-19395-8</isbn>
        </book>
        <bicycle price="19.95" d:price="Number" d:mi="21">
            <brand d:constr="String" d:mi="19">Cannondale</brand>
            <color d:constr="String" d:mi="20">red</color>
        </bicycle>
    </store>
</d:data>

result.xml

<?xml version="1.0" encoding="utf-8"?>
<prfx1:data xmlns:prfx1="defiant-namespace" prfx1:mi="23">
  <store prfx1:mi="22">
    <book prfx1:price="Number" prfx1:mi="4" price="12.99">
      <title prfx1:mi="1" prfx1:constr="String">Sword of Honour</title>
      <category prfx1:mi="2" prfx1:constr="String">fiction</category>
      <author prfx1:mi="3" prfx1:constr="String">Evelyn Waugh</author>
    </book>
    <book prfx1:price="Number" prfx1:mi="9" price="8.99">
      <title prfx1:mi="5" prfx1:constr="String">Moby Dick</title>
      <category prfx1:mi="6" prfx1:constr="String">fiction</category>
      <author prfx1:mi="7" prfx1:constr="String">Herman Melville</author>
      <isbn prfx1:mi="8" prfx1:constr="String">0-553-21311-3</isbn>
    </book>
    <foo /> <!-- Here is changed -->
    <book prfx1:price="Number" prfx1:mi="18" price="22.99">
      <title prfx1:mi="14" prfx1:constr="String">The Lord of the Rings</title>
      <category prfx1:mi="15" prfx1:constr="String">fiction</category>
      <author prfx1:mi="16" prfx1:constr="String">J. R. R. Tolkien</author>
      <isbn prfx1:mi="17" prfx1:constr="String">0-395-19395-8</isbn>
    </book>
    <bicycle prfx1:price="Number" prfx1:mi="21" price="19.95">
      <brand prfx1:mi="19" prfx1:constr="String">Cannondale</brand>
      <color prfx1:mi="20" prfx1:constr="String">red</color>
    </bicycle>
  </store>
</prfx1:data>

sxslt.scm

(import (rnrs)
	(text sxml sxpath)
	(text sxml ssax)
	(text sxml serializer)
	(srfi :26))

(define sxml (call-with-input-file "foo.xml" (cut ssax:xml->sxml <> '())))

(define (sxslt sxml)
  (lambda (xpath . ns-binding)
    (define path (apply sxpath xpath ns-binding))
    (define converted (make-eq-hashtable))
    ;; proc: sxml -> sxml
    (lambda (proc)
      (for-each (lambda (child)
		  (hashtable-set! converted child (proc child)))
		(path sxml))
      (let loop ((sxml sxml))
	(cond ((null? sxml) '())
	      ((pair? sxml)
	       (cond ((hashtable-ref converted sxml #f))
		     (else (cons (loop (car sxml)) (loop (cdr sxml))))))
	      (else
	       (cond ((hashtable-ref converted sxml #f))
		     (else sxml))))))))

(let ((c ((sxslt sxml) "//*[title='50']")))
  (display (srl:sxml->xml (c (lambda (sxml) '(foo))))))