how do I get the PI to appear at the beginning of the document?

      my $xsl = new XML::Twig::Elt('#PI');
      $xsl->set_target('xml-stylesheet');
      $xsl->set_data('type= "text/xls" href="xls_style.xls"');
      $xsl->paste('before', $twig->root);
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This fails because you can't paste before the root.