keep track of items (of type <code>T</code>)
in a thread-safe way.</p>

<p>The lifetime of an item in the registry is determined by the <code><a href="Protocols/Receipt.html">Receipt</a></code> that
was returned when the item was registered.</p>

<p>You must maintain at least one strong reference to the returned <code><a href="Protocols/Receipt.html">Receipt</a></code> for
the duration of the time that you want the item it represents to remain
registered. Once the <code><a href="Protocols/Receipt.html">Rece