Create recursive markup with MarkupBuilder

I just found a way use Groovy's MarkupBuilder to build an XML file with nested elements. Assuming I have a container with a collection of X objects and a Y object. The XML output of the container can be generated as follows:

import groovy.xml.*

class X {
  private int id
  private String name

  def toXML = { builder ->
    builder.x(id: id, name)
  }
}

class Y {
  private Date date = new Date()
  def toXML = { builder ->
    final sdf = new java.text.SimpleDateFormat("yyyyMMdd'T'HH:mm:ssZ")
    builder.y(date: sdf.format(date))
  }
}

def w = new StringWriter()
def xml = new MarkupBuilder(w)
xml.doubleQuotes = true
def xa = [ new X(id: 1, name: "x-name"), new X(id: 2, name: 'x-name2') ]
def yObj = new Y()
xml.root() {
  agent(id: 1, name: 'agent1', "X")
  xa.each { xo ->
    xo.toXML(xml)
  }
  yObj.toXML(xml)
}
w.close()

The variable w is equivalent to:

<root>
 <agent id="1" name="agent1">X</agent>
 <x id="1">x-name</x>
 <x id="2">x-name2</x>
 <y date="20080318T23:56:54+0800" />
</root>

Notice that to get the builder to output double quotes for attributes, I set the doubleQuotes property to true.