Groovy add to XML before saving

How can I add to XML something like this? I know, that there are http://ift.tt/Xu8QZm and http://ift.tt/1tw0NrL but, I don't need it. I will have something like this:

<logger>
  <config name="test1" level="ERROR">
    <pack>com.simple.test</pack>
    ...
  </config>
  <config name="test2" level="INFO">
    <pack>com.simple.test</pack>
    ...
</config>
<config name="test2" level="INFO">
...
</logger>

I have loop in loop. One loop create one config with name and level, the second loop adds pack with strings like "com.simple.test" And I will do this only once. So I think, that no sense to create file, fill it, close, open, update, close, update... and it will do this loop1_steps * loop2_steps. Now I'm doing this that way, but don't understend how to update it before saving file. So, I want first create full xml, and save it next.

import groovy.xml.MarkupBuilder

def writer = new FileWriter(new File( "C:\\dev\\test.xml" ))
def xml = new MarkupBuilder(writer)
xml.logger() {
    config(name:'test1', level:'ERROR') {
        pakiet('com.simple.test')
    }
    config(name:'test2', level:'INFO') {
        pack('com.simple.test')

    }
}

Thanks!