// Make sure panic during callback unwinds properly.
	if lockedOSThread() {
		t.Fatal("locked OS thread on entry to TestCallbackPanic")
	}
	defer func() {
		s := recover()
		if s == nil {
			t.Fatal("did not panic")
		}
		if s.(string) != "callback panic" {
			t.Fatal("wrong panic:", s)
		}
		if lockedOSThread() {
			t.Fatal("locked OS thread on exit from TestCallbackPanic")
		}
	}()

to define an interface called <code>Locker</code>:
</p>

<pre>
type Locker interface {
	Lock()
	Unlock()
}
</pre>

<p>
If <code>S1</code> and <code>S2</code> also implement
</p>

<pre>
func (p T) Lock() { … }
func (p T) Unlock() { … }
</pre>

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