on allows logic to be shared among rules.  It also allows easy use of boolean operators such as <em>and</em> and <em>or</em> to compose logic.</p>
</dd>
<dt class="hdlist1">Performance</dt>
<dd>
<p>Common subexpressions are merged, including across rules in the same phase.  Such merging can have significant performance benefits, especially in rule sets with significant common logic.  In addition, evaluation that can be done at configuration time is done so.  This pre-evaluation is especially useful for impl