Get the "select" attribute.
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.