The ID/IDREF mechanism for locating elements in XML documents has been
generalized in XSL to the notion of keys.
ID/IDREF is only useful when:
The document has declarations that identify the ID and IDREF
The processor is capable of processing the declarations
Using select expressions (XPath) to locate elements may be
Declaring keys gives the stylesheet processor an indication of what
elements should be cached for fast access.
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Sun Microsystems, Inc. (http://www.sun.com/)