hen each
AUTHOR child of the matched element supplies one key value. If the expression returns any other value, the value
is converted to a string and that string acts as the key.</p>

<p>Note that<ol>
<li>Keys are not unique: the same value may identify many different nodes</li>
<li>Keys are multi-valued: each matched node may have several (zero or more) values of the key, any one
of which may be used to locate that node</li>
<li>Keys can only be used to identify nodes within a single XML document: