Root[1] vs Root

[Branko N. <bra...@in...>]

I've noticed that root absolute path need to be numbered as root[1]:
    $set  =  $x1->match("//document", "/root[1]/folder");

In case without [1] after root
    $set  =  $x1->match("//document", "/root/folder");
have got error:
*XPath error in XPath.class.php:2121* The supplied xPath '//document' 
does not *uniquely* describe a node in the xml document.

Seems strange to me, as root is always the only one.

Branko