ode>AddOrderItem</code>) by
    </p>
    <ul>
        <li>verifying that the command is valid and can be accepted and in case it is</li>
        <li>emitting an event (or multiple) that describes the state-change (by calling <code>persist(event)</code>)</li>
    </ul>
    <p>
        So for example if an <code>AddOrderItem</code>-command can be accepted the
        <code>onCommand</code>-handler emits an <code>OrderItemAdded</code>-event.
        Before this event is processed by the <code>onEvent</code>-ha