diff --git a/util/aop.py b/util/aop.py
index 65f80d4..a66a1f6 100644
--- a/util/aop.py
+++ b/util/aop.py
@@ -34,13 +34,22 @@ class Notifier(object):
     def __init__(self, method):
         self.method = method
         obj = method.im_self or method.im_class
-        name = method.im_func.func_name
+        name = self.name = method.im_func.func_name
         setattr(obj, name, self)
         self.beforeSubs = []
         self.afterSubs = []
 
     def __get__(self, instance, cls=None):
-        return BoundNotifier(self, instance)
+        if instance is None:
+            # class-level access
+            return self
+        existing = instance.__dict__.get(self.name)
+        if isinstance(existing, BoundNotifier):
+            # the instance's method is already wrapped
+            return existing
+        notifier = BoundNotifier(self, instance)
+        setattr(instance, self.name, notifier)
+        return notifier
 
     def __call__(self, *args, **kw):
         for sub, sargs, skw in self.beforeSubs:
@@ -62,15 +71,21 @@ class BoundNotifier(object):
     def __init__(self, context, instance):
         self.context = context
         self.instance = instance
+        self.beforeSubs = []
+        self.afterSubs = []
 
     def __call__(self, *args, **kw):
-        for sub, sargs, skw in self.context.beforeSubs:
+        context = self.context
+        for sub, sargs, skw in self.beforeSubs + context.beforeSubs:
             sub(None, *sargs, **skw)
-        result = self.context.method(self.instance, *args, **kw)
-        for sub, aargs, akw in self.context.afterSubs:
+        result = context.method(self.instance, *args, **kw)
+        for sub, aargs, akw in self.afterSubs + context.afterSubs:
             sub(result, *aargs, **akw)
         return result
 
     def subscribe(self, before, after, *args, **kw):
-        self.context.subscribe(before, after, *args, **kw)
+        if before is not None:
+            self.beforeSubs.append((before, args, kw))
+        if after is not None:
+            self.afterSubs.append((after, args, kw))
 
diff --git a/util/aop.txt b/util/aop.txt
index b96d0a0..562faaa 100644
--- a/util/aop.txt
+++ b/util/aop.txt
@@ -28,8 +28,8 @@ Calling the wrapped method still works.
   >>> demo.foo(8)
   46
 
-We can now get information about the returning of the method by
-subscribing to it using some log function.
+We can now get information about the start and end of the method call by
+subscribing to it using a logging routine.
 
   >>> def log(result, msg):
   ...     print 'logging: %s, result=%s' % (msg, str(result))
@@ -51,14 +51,36 @@ subscribing to it using some log function.
 Wrapping methods on class level
 ===============================
 
+The above code did only wrap the bound method ``foo()`` of a certain
+instance of the ``Demo`` class. Other instances of this class are not
+affected.
+
   >>> demo = Demo()
   >>> demo.foo(4)
   42
 
+But we may also wrap the class itself and subscribe to the method on
+the class level.
+
   >>> getNotifier(Demo.foo)
   <...Notifier object ...>
   >>> Demo.foo.subscribe(None, log, 'after foo')
+
+This now affects all instances of the class.
+
   >>> demo.foo(4)
   logging: after foo, result=42
   42
+  >>> demo.foo(8)
+  logging: after foo, result=46
+  46
+
+Combining class and instance level wrapping
+-------------------------------------------
+
+  >>> demo.foo.subscribe(None, log, 'after demo.foo')
+  >>> demo.foo(10)
+  logging: after demo.foo, result=48
+  logging: after foo, result=48
+  48