[NTG-context] Extensible arrows in ConTeXT

Dear list, I want to typeset an extensible arrow as described in this document: http://dl.contextgarden.net/myway/matharrows.pdf http://dl.contextgarden.net/myway/matharrows.pdf Sadly I get errors if I use \xrightarrow (even this simple command does not work). Do extensible arrows still exists in ConTeXT and how can I use them? Best Mathias