<configure
    xmlns="http://namespaces.zope.org/zope"
    xmlns:five="http://namespaces.zope.org/five">

    <subscriber
        for="Products.CMFPlone.interfaces.IPloneSiteRoot
             zope.app.publication.interfaces.IBeforeTraverseEvent"
        handler=".layer.mark_layer"
        />

</configure>
