[xsl] use of $foo/(//path)

In this article:

http://www.stylusstudio.com/schema_aware.html

I noticed this:

<xsl:sequence select="$actor/(//video[actorRef=$actor/@id])"/>

The part which caught my eye was:

$actor/(//...)

What does the sequence constructor do for you here, as opposed to just:

$actor//...

?

thanks
andrew