Precompiling Xpath expressions

Hi all.
Probably there might be post for this in past, but I didnt find one such. 
What is exactly precompiled Xpath expression and how to do this with Apache Xalan-Xerces API. 
Somebody mentioned that its discussed under "Usage Patters" of xalan docs. But i dont see any such topic.
Pl somebody give me some clue.

thanks in advance
Venkat

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