[Typo3-dev] Need help extending a system class

[Jochen Weiland]

I need to manipulate all links that are generated by the typolink 
function in Typo3. I thought the best way would be to write an extension 
class for the typolink function (similar to the example that manipulates 
the title string):

require_once (PATH_tslib."class.tslib_content.php");

class ux_txslib_content extends tslib_content {...}

But this always results in the error:

Fatal error: Class ux_tslib_content: Cannot inherit from undefined class 
tslib_content

Any idea what I am doing wrong? Should be quite a simple mistake that I 
made...

Regards
Jochen