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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.  |  
 
 
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Returns: 
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the associated namespace URI or null if none is found. 
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