<xsl:variable name="foo" as="element(foo)+">
 <foo/>
 <foo/>
 <foo/>
</xsl:variable>

<xsl:for-each select="$foo">
 <xsl:value-of select="count(preceding-sibling::*)"/>
</xsl:for-each>

I seem to remember David C mentioning something about using
as="element()" means the elements are no longer siblings... but I
can't find anything about it now.

thanks
andrew