<xsl:key> element declares a named key which can be used elsewhere in the stylesheet with the
key( ) function.
<xsl:key name=NAME match=EXPRESSION use=EXPRESSION />
- Specifies a name for this key. Must be a QName.
- Defines the nodes for which this key is applicable.
- Specifies an XPath expression that will be used to determine the value of the key for each of the applicable nodes.
Top-level, must be the child of
XSLT, section 12.2.