Insert a tag with NSFileHandle

Question

Hi, I want insert a tag in the Xml file. I have NSFileHandle object and i know the "offsetInFile" and the "seekToFileOffset:" methods but i don't know how to use. I have the Xml below and i want insert a tag between "positions" and "/positions" tag. Thanks so much.

<?xml version=1.0 encoding=UTF-8 ?>
<positions>

 //I want insert in this position my tag with the NSFileHandle.

</positions>