key function returns a node-set of nodes that have the given value for the given key.
key(keyname ,value )
A string containing the name of the
xsl:keyelement to be used.
The returned node-set will contain every node that has this value for the given key.
A node set.
xsl:keyelement defines what attribute on what given elements will be used to match the key.
This function is an XSLT-specific addition to XPath. It is not a part of the core XPath function library.