tional -both probe for the named property being defined.
 The <code>if</code> tests for the property being defined, the
 <code>unless</code> for a property being undefined.

 If both attributes are set, then the test fails only if both tests
 are true. i.e.
 <pre>fail := defined(ifProperty) &amp;&amp; !defined(unlessProperty)</pre>

 A single nested<code>&lt;condition&gt;</code> element can be specified
 instead of using <code>if</code>/<code>unless</code> (a combined
 effect can be achieved using <code>iss