The other day, "sibling recursion" was mentioned on this list.
Does the following qualify as sibling recursion - and is this
likely to be more efficient for large input?

       <xsl:apply-templates mode="toc"
         select="(following-sibling::h1 |
following-sibling::h2)[ 1 ]"/>

Might be safer to do following-sibling::*[self::h1 or self::h2][1] -
but depends on the processor.

Why might this be safer? Aren't the resulting node-sets the same and
the expressions therefore equivalent? Doesn't the union operator impose
document order?