.</dd>
        <dt>Reduce expression</dt>
        <dd>A JSONata expression that is called for each message in the sequence.
            The result is passed to the next call of the expression as the accumulated value.
            In the expression, the following special variables can be used:
            <ul>
                <li><code>$A</code>: the accumulated value, </li>
                <li><code>$I</code>: index of the message in the sequence, </li>
                <li><code>$N</code>: number of message