aria -- how much can be inferred?

http://www.w3.org/TR/wai-aria/#mustContain

    Any element that must be owned by the element with this role.
    For example, an element with the role list must own an element
    with the role listitem.

How explicit does this child role have to be?

(a)  Is it enough that the native semantics imply the child role?  (I
assume so.)
(b)  Is it enough that the native semantics plus validity imply the
child role?  (Not as sure.)
For example, in

    <ul role=menu>
        <li role="menuitem">Open file…</li>

is the <li> role optional, because host language semantics make it a
listitem, and the containing list is known to have the menu role?

-jJ