xml
<solution>Inhalt<valid_input>Inhalt</valid_input><valid_input>
Inhalt_1</valid_input></solution>
xsl
<xsl:variable name="total"
select="count(solution/valid_input[parent::solution =
self::valid_input])" />
If you are trying to count all the valid_input elements you just need
count(solution/valid_input)
If you are only trying to count some of them, you will need to explain
which ones, because your predicate is completely bizarre: it selects
those valid_input elements that have the same string value as their
parent node.
Michael Kay
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list