The key() function finds nodes in which a named key has a given value. It is used with the <xsl:key> element.
node-set key(string, value)
The name of the key and the value to be matched. If the value is not a node-set, it is converted into a string using the rules for the
string() function. If the argument is a node-set, the
key() is applied to the string value produced when each node in the node-set is converted into a string.
A node-set, containing all matching nodes from the same document as the context node.
Defined: XSLT, section 12.2.
