<p>
they implement the <code>Locker</code> interface as well
as the <code>File</code> interface.
</p>

<p>
An interface <code>T</code> may use a (possibly qualified) interface type
name <code>E</code> in place of a method specification. This is called
<i>embedding</i> interface <code>E</code> in <code>T</code>.
The <a href="#Method_sets">method set</a> of <code>T</code> is the <i>union</i>