my $xml_fragment = XML::Struct->new( <<EOF ); # fictitious...
        <records>
          <record id="001">foo</record>
          <record id="002">bar</record>
        </records>
EOF
    my @nodeset = $xml_fragment->xQuery( "//records[@id = '001']" );
    die "ids should be unique!\n" if @nodeset > 1;