XPath: How to exclude depending on href?

[Ranhiru]

Im writing a Grease Monkey script to beautify one of the websites i use!
It is a bit messy with all the anchor <a> tags underlined! I want to set the text-decoration to none in links with href to the same website.

That means all links connecting to external websites should have their link underlined, while text-decoration of links to threads, posts of the same website are removed

This is what i came up with

allLinks = document.evaluate(
    '//a[@href=starts-with(name(),www.mywebsite.com)]', 
    document,
    null,
    XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE,
    null);

Well this nicely clears all the underlines in the <a> tags BUT including links to other websites too :(
How do make sure that <a> tags of mysite only get the text-decoration to none?