Inner ViewMdel cannot get Outer ViewModel in Binding

<?page title="Nested ViewModel" contentType="text/html;charset=UTF-8"?>
<div apply="org.zkoss.bind.BindComposer"
viewModel="@id('vm') @init('org.zkoss.zktest.bind.include.FirstViewModel')">

<div p="@ref(vm.selected)" pid="@ref(vm.selected.id)" >

<label id="sid" value="@load(pid)" ></label>
<label value="@load(p.someList)" ></label>

<div apply="org.zkoss.bind.BindComposer"
viewModel="@id('vms') @init('org.zkoss.zktest.bind.include.SecondViewModel')">

pid won't work:
<label id="sid1" value="@load(pid)" ></label>
p.someList won't work:
<label value="@load(p.someList)" ></label>
vm won't work:
<label value="@load(vm)" ></label>
</div>
</div>
</div>