xsl:sort has a select attribute whose value is an expression. For each node to be processed, the expression is evaluated with
that node as the current node and with the complete list of nodes being processed in unsorted order as the current node list.
The resulting object is converted to a string as if by a call to the string function; this string is used as the sort key
for that node. The default value of the select attribute is ., which will cause the string-value of the current node to be
used as the sort key.
Examples (xslt:xml): Overview 19:5