Modifying XML file structure (Groovy)

import org.dom4j.io.*
import org.dom4j.*
 
new File('in/emissions').eachFile{ file ->
 
    if (!file.name.endsWith('.xml'))
	return
 
    def emission  = new SAXReader().read(file).rootElement
 
    def sondage = emission.addElement('pr:sondage')
    sondage.addAttribute('xsi:nil', 'true')
    sondage.setText('n/a')
 
    def writer = new FileWriter('out/' + file.name)
    XMLWriter output = new XMLWriter(writer, OutputFormat.createPrettyPrint())
 
    output.writeNode(emission)
    output.close()
}