position()

An integer equal to the context position from the expression evaluation context.

• Note that a node's position in a context is not zero-based. The first node has a position of 1.
• The context is determined by the rest of the path.

  <xsl:template match="//a[position() = 5]">
    <!-- this template matches the fifth a element
            anywhere in the document. -->
  </xsl:template>
  

  <xsl:template match="//div[@class='foo']/bar[position() = 1]">
    <!-- this template matches the first bar element that is
        a child of a div element with a class attribute equal to "foo" -->
  </xsl:template>
  

XPath 1.0 4.1

Supported.