Content is not allowed in prolog

This one is rather tricky, while transforming xml with xslt with this jstl:
transform.jsp

  <%@ page session="false" contentType="text/html; charset=utf-8"%> 
  <%@ taglib prefix="c"  uri="http://java.sun.com/jsp/jstl/core" %>
  <%@ taglib prefix="x"  uri="http://java.sun.com/jsp/jstl/xml"  %>
  <x:transform xslt="question.xsl">
    <c:import url="questions.jspx" />
  </x:transform>

I've got this error:

javax.xml.transform.TransformerException:
org.xml.sax.SAXParseException: Content is not allowed in prolog."


The solution is to specify the xsl-file in a variable:

  <%@ page session="false" contentType="text/html; charset=utf-8"%>
  <%@ taglib prefix="c"  uri="http://java.sun.com/jsp/jstl/core" %>
  <<@ taglib prefix="x"  uri="http://java.sun.com/jsp/jstl/xml"  %>
  <c:import url="question.xsl" var="xslfile"/>
  <x:transform xslt="${xslfile}">
    <c:import url="questions.jspx" />
  </x:transform>