Any element can be a link.
An xmlns:xlink attribute associates the xlink
prefix with the http://www.w3.org/1999/xlink namespace.
XLink elements are identified by an xlink:type attribute with
one of these six values:
simple
extended
locator
arc
resource
title
Linking elements are identified by an xlink:type attribute with
one of these two values:
simple
extended
xlink:href attributes
contain the URI of the resource being linked to.