Tag variables

Element names can also be bound by variables:

  WHERE <$p>
          <title> $t </title>
          <year> 1995 </>
          <$e> Smith </>
        </> IN "www.a.b.c/bib.xml",
        $e IN {author, editor}
  CONSTRUCT <$p>
              <title> $t </title>
              <$e> Smith </>
            </>

$p matches book and article.
$e matches author and editor.
this saves us from writing four queries.

This finds all publications in 1995 where Smith is either author or editor.