 <code>PolicyContext</code> handler. A handler may be registered
 to handle multiple keys, but at any time, at most one handler may be
 registered for a key.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>key</code> - a (case-sensitive) <code>String</code> that identifies
 the context object handled by the handler.
 The value of this parameter must not be null.</dd>
<dd><code>handler</code> - an object that implements the 
 <code>PolicyContextHandler</code> interface. The value of