e set of nodes to which the key applies is defined by a pattern in the match attribute: for example,
if match="ACT|SCENE" then every ACT element and every SCENE element is indexed by this key.</p>

<p>The value of the key, for each of these matched elements, is determined by the <b>use</b> attribute.
This is an expression, which is evaluated for each matched element. If the expression returns a node-set,
the string value of each node in this node-set acts as a key value. For example, if use="AUTHOR", t