equality
xpath \
xpath \\ DESCENDANTS
<book><author>Peter Buneman</author><author>Dan Suciu</author><title>Data on ze web</title></book>
-- group nodes
<f><a></a><b></b><c></c></f>
<a></a><f><a></a><b></b><c></c></f><a></a><b></b><c></c>
attribute value normalization
