diff --git a/src/wormhole/observer.py b/src/wormhole/observer.py
index 43afa70..34fbf1b 100644
--- a/src/wormhole/observer.py
+++ b/src/wormhole/observer.py
@@ -70,3 +70,24 @@ class SequenceObserver(object):
             if self._observers:
                 d = self._observers.pop(0)
                 self._eq.eventually(d.callback, self._results.pop(0))
+
+
+class EmptyableSet(set):
+    # manage a set which grows and shrinks over time. Fire a Deferred the first
+    # time it becomes empty after you start watching for it.
+
+    def __init__(self, *args, **kwargs):
+        self._eq = kwargs.pop("_eventual_queue")  # required
+        super(EmptyableSet, self).__init__(*args, **kwargs)
+        self._observer = None
+
+    def when_next_empty(self):
+        if not self._observer:
+            self._observer = OneShotObserver(self._eq)
+        return self._observer.when_fired()
+
+    def discard(self, o):
+        super(EmptyableSet, self).discard(o)
+        if self._observer and not self:
+            self._observer.fire(None)
+            self._observer = None
diff --git a/src/wormhole/test/test_observer.py b/src/wormhole/test/test_observer.py
index 8d89ddd..6ca8957 100644
--- a/src/wormhole/test/test_observer.py
+++ b/src/wormhole/test/test_observer.py
@@ -3,7 +3,7 @@ from twisted.python.failure import Failure
 from twisted.trial import unittest
 
 from ..eventual import EventualQueue
-from ..observer import OneShotObserver, SequenceObserver
+from ..observer import OneShotObserver, SequenceObserver, EmptyableSet
 
 
 class OneShot(unittest.TestCase):
@@ -121,3 +121,28 @@ class Sequence(unittest.TestCase):
         d2 = o.when_next_event()
         eq.flush_sync()
         self.assertIdentical(self.failureResultOf(d2), f)
+
+
+class Empty(unittest.TestCase):
+    def test_set(self):
+        eq = EventualQueue(Clock())
+        s = EmptyableSet(_eventual_queue=eq)
+        d1 = s.when_next_empty()
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        s.add(1)
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        s.add(2)
+        s.discard(1)
+        d2 = s.when_next_empty()
+        eq.flush_sync()
+        self.assertNoResult(d1)
+        self.assertNoResult(d2)
+        s.discard(2)
+        eq.flush_sync()
+        self.assertEqual(self.successResultOf(d1), None)
+        self.assertEqual(self.successResultOf(d2), None)
+
+        s.add(3)
+        s.discard(3)