Get the position of an element with specific attribute value

I’m trying to get with xPath the position only of the first element which has the attribute value true.

<?xml version="1.0" encoding="UTF-8"?>
<elements>
    <element attribute="false"/>
    <element attribute="true"/>
    <element attribute="true"/>
</elements>

What I have so fare is:

 head(/elements/element[@attribute='true']/position())

Result:

 1

But it should be:

 2

What am I doing wrong?

>Solution :

position() returns the position of the element in the nodelist created by the predicate, i.e. with the false excluded. Instead of position, you can e.g. count the number of preceding elemements.

For example, this works even in XPath 1.0:

1+count(/elements/element[@attribute="true"][1]/preceding-sibling::element)