children.indexOf(page);
};

// returns a sibling page, or the nullObject-page if no sibling exists at the moment
var pageStep = function(page, step) {
    return ko.computed(function() {
        var rawStep = ko.utils.unwrapObservable(step);
        return page.parentPage.children()[(indexOfCurrentPage(page)) + rawStep] || page.nullObject;
    });
};
</pre>
<p>
    that now can be used in a new class:
</p>

<pre data-bind="prettyprint: {}" class="prettyprint linenums">
// new constructor
var NextPage = func