xpath: stop at first match

Hi,

Forgive me if this message seems a bit off-topic. It does however pertain 
to a topic maps application I'm developing.
I've looked around around quite a bit, but I can't seem to determine 
whether there is an xpath expression construct that would cause the 
processor to stop upon finding the first match (as opposed to always 
scanning the whole document for all possible matches).
Do anyone have some info to shed on this? And if the construct doesn't 
exist, does it strike you as a quite desirable optimization?
Thank you,
Carlo Moneti
http://arsteca.net