<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.