 <p>By default, the reduce expression is applied in order, from the first
           to the last message of the sequence. It can optionally be applied in
           reverse order.</p>
        <p>$N is the number of messages that arrive - even if they are identical.</p>
    </dl>
    <p><b>Example:</b> the following settings, given a sequence of numeric values,
       calculates the average value:
        <ul>
            <li><b>Reduce expression</b>: <code>$A+payload</code></li>
            <li><b>Initial v