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Look up the namespace URI associated to the given namespace prefix. 
 
The XPath evaluator must never call this with a null or empty argument, because the result of doing this is undefined.  - Parameters:
 - 
  
     | prefix  | of type XMLCh - The prefix to look for.  |  
   
  
- Returns:
 - the associated namespace URI or null if none is found. 
  
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