diff --git a/tracking/btree.py b/tracking/btree.py
index ccdc7af..04f580b 100644
--- a/tracking/btree.py
+++ b/tracking/btree.py
@@ -1,5 +1,5 @@
 #
-#  Copyright (c) 2007 Helmut Merz helmutm@cy55.de
+#  Copyright (c) 2008 Helmut Merz helmutm@cy55.de
 #
 #  This program is free software; you can redistribute it and/or modify
 #  it under the terms of the GNU General Public License as published by
@@ -17,7 +17,7 @@
 #
 
 """
-BTree-based implementation of user interaction tracking.
+ZODB-/BTree-based implementation of user interaction tracking.
 
 $Id$
 """
@@ -84,6 +84,7 @@ class Track(Persistent):
         return '<Track %s: %s>' % (`[md[a] for a in self.metadata_attributes]`,
                                      `self.data`)
 
+
 class TrackingStorage(BTreeContainer):
 
     implements(ITrackingStorage)
@@ -93,10 +94,13 @@ class TrackingStorage(BTreeContainer):
     trackNum = runId = 0
     runs = None
 
-    #indexAttributes = ('taskId', 'runId', 'userName', 'timeStamp')
     indexAttributes = Track.index_attributes
 
     def __init__(self, *args, **kw):
+        trackFactory = kw.pop('trackFactory', None)
+        if trackFactory is not None:
+            self.trackFactory = trackFactory
+            self.indexAttributes = trackFactory.index_attributes
         super(TrackingStorage, self).__init__(*args, **kw)
         self.indexes = OOBTree.OOBTree()
         for idx in self.indexAttributes:
@@ -117,10 +121,11 @@ class TrackingStorage(BTreeContainer):
     def idFromNum(self, num):
         return '%07i' % (num)
 
-    def startRun(self, taskId):
+    def startRun(self, taskId=None):
         self.runId += 1
         runId = self.runId
-        self.currentRuns[taskId] = runId
+        if taskId is not None:
+            self.currentRuns[taskId] = runId
         run = self.runs[runId] = Run(runId)
         run.start = run.end = getTimeStamp()
         return runId
@@ -147,6 +152,11 @@ class TrackingStorage(BTreeContainer):
             return self.runs.get(runId)
         return None
 
+    def generateTrackId(self):
+        self.trackNum += 1
+        trackId = self.idFromNum(self.trackNum)
+        return trackId, self.trackNum
+
     def saveUserTrack(self, taskId, runId, userName, data, update=False):
         if not runId:
             runId = self.currentRuns.get(taskId) or self.startRun(taskId)
@@ -154,15 +164,11 @@ class TrackingStorage(BTreeContainer):
         if run is None:
             raise ValueError('Invalid run: %i.' % runId)
         run.end = getTimeStamp()
-        trackNum = 0
         if update:
             track = self.getLastUserTrack(taskId, runId, userName)
             if track is not None:
                 return self.updateTrack(track, data)
-        if not trackNum:
-            self.trackNum += 1
-            trackNum = self.trackNum
-            trackId = self.idFromNum(trackNum)
+        trackId, trackNum = self.generateTrackId()
         track = self.trackFactory(taskId, runId, userName, data)
         self[trackId] = track
         self.indexTrack(trackNum, track)
@@ -211,12 +217,10 @@ class TrackingStorage(BTreeContainer):
             if idx in self.indexAttributes:
                 if type(value) not in (list, tuple):
                     value = [value]
-                # TODO: handle a list of values, provide union of results
                 resultx = None
                 for v in value:
                     resultx = self.union(resultx, self.indexes[idx].apply((v, v)))
                 result = self.intersect(result, resultx)
-                #result = self.intersect(result, self.indexes[idx].apply((value, value)))
             elif idx == 'timeFrom':
                 result = self.intersect(result,
                                         self.indexes['timeStamp'].apply((value, None)))
diff --git a/tracking/comment/README.txt b/tracking/comment/README.txt
new file mode 100644
index 0000000..06e5e38
--- /dev/null
+++ b/tracking/comment/README.txt
@@ -0,0 +1,10 @@
+=============
+Notifications
+=============
+
+  ($Id$)
+
+  >>> from cybertools.tracking.btree import TrackingStorage
+  >>> from cybertools.tracking.notify.base import Notification
+
+  >>> comments = TrackingStorage(trackFactory=Notification)
diff --git a/tracking/comment/__init__.py b/tracking/comment/__init__.py
new file mode 100644
index 0000000..4bc90fb
--- /dev/null
+++ b/tracking/comment/__init__.py
@@ -0,0 +1,4 @@
+"""
+$Id$
+"""
+
diff --git a/tracking/comment/base.py b/tracking/comment/base.py
new file mode 100644
index 0000000..9318e7f
--- /dev/null
+++ b/tracking/comment/base.py
@@ -0,0 +1,33 @@
+#
+#  Copyright (c) 2008 Helmut Merz helmutm@cy55.de
+#
+#  This program is free software; you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation; either version 2 of the License, or
+#  (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software
+#  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+
+"""
+Basic implementation of comments / discussions.
+
+$Id$
+"""
+
+from zope.interface import implements
+
+from cybertools.tracking.btree import Track
+from cybertools.tracking.comment.interfaces import IComment
+
+
+class Comment(Track):
+
+    implements(IComment)
diff --git a/tracking/comment/interfaces.py b/tracking/comment/interfaces.py
new file mode 100644
index 0000000..be9b750
--- /dev/null
+++ b/tracking/comment/interfaces.py
@@ -0,0 +1,73 @@
+#
+#  Copyright (c) 2008 Helmut Merz helmutm@cy55.de
+#
+#  This program is free software; you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation; either version 2 of the License, or
+#  (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software
+#  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+
+"""
+Interface definitions for comments - discussions - forums.
+
+$Id$
+"""
+
+from zope.interface import Interface, Attribute
+from zope import schema
+
+from cybertools.tracking.interfaces import ITrack
+
+
+class IComment(ITrack):
+    """ A comment is a piece of text provided by a user and related
+        to a content object.
+        The object the comment is related to is referenced via the
+        task id attribute; interdependent comments (i.e. comments in a
+        parent/child hierarchy related to the same object share the
+        run id; the user name references the user/person that created
+        the comment.
+    """
+
+    parent = Attribute('The id of the parent comment; None for '
+                'the top-level comment.')
+
+    subject = schema.TextLine(
+        title=u'Subject',
+        description=u'A short informative line of text.',
+        default=u'',    # should be taken from the parent
+        required=True)
+    text = schema.Text(
+        title=u'Text',
+        description=u'The text of the comment.',
+        default=u'',
+        required=True)
+    contentType = schema.BytesLine(
+        title=u'Content Type',
+        description=u'Content type (format) of the text field',
+        # TODO: provide a source/vocabulary
+        default='text/restructured',
+        required=True)
+
+    def getChildren(sort='default'):
+        """ Returns the immediate children in the order specified by the sort
+            argument; the return value is a sequence of track ids.
+            The default sorting is by timestamp, newest first.
+        """
+
+    def getChildrenTree(sort='default', depth=0):
+        """ Returns the children as a tree, in the order specified by
+            the sort argument. If depth is greater than 0 only that level
+            of children will be returned.
+            The return value is a nested sequence of track ids.
+        """
+
diff --git a/tracking/comment/tests.py b/tracking/comment/tests.py
new file mode 100755
index 0000000..6c7ef8b
--- /dev/null
+++ b/tracking/comment/tests.py
@@ -0,0 +1,22 @@
+# $Id$
+
+import unittest, doctest
+from zope.testing.doctestunit import DocFileSuite
+
+
+class Test(unittest.TestCase):
+    "Basic tests for the cybertools.tracking.notify package."
+
+    def testBasics(self):
+        pass
+
+
+def test_suite():
+    flags = doctest.NORMALIZE_WHITESPACE | doctest.ELLIPSIS
+    return unittest.TestSuite((
+                unittest.makeSuite(Test),
+                DocFileSuite('README.txt', optionflags=flags),
+            ))
+
+if __name__ == '__main__':
+    unittest.main(defaultTest='test_suite')
diff --git a/tracking/interfaces.py b/tracking/interfaces.py
index 162924e..249b3cf 100644
--- a/tracking/interfaces.py
+++ b/tracking/interfaces.py
@@ -1,5 +1,5 @@
 #
-#  Copyright (c) 2007 Helmut Merz helmutm@cy55.de
+#  Copyright (c) 2008 Helmut Merz helmutm@cy55.de
 #
 #  This program is free software; you can redistribute it and/or modify
 #  it under the terms of the GNU General Public License as published by
@@ -17,7 +17,7 @@
 #
 
 """
-loops tracking interface definitions.
+Interface definitions for tracking of user interactions.
 
 $Id$
 """
@@ -51,8 +51,9 @@ class ITrackingStorage(Interface):
     """ A utility for storing user tracks.
     """
 
-    def startRun(taskId):
-        """ Create a new run for the task given and return its id.
+    def startRun(taskId=None):
+        """ Create a new run and return its id.
+            If a taskId is given, record the runId as the tasks current run.
         """
 
     def stopRun(taskId=None, runId=0, finish=True):
diff --git a/tracking/notify/README.txt b/tracking/notify/README.txt
new file mode 100644
index 0000000..8fd7fc7
--- /dev/null
+++ b/tracking/notify/README.txt
@@ -0,0 +1,10 @@
+========
+Comments
+========
+
+  ($Id$)
+
+  >>> from cybertools.tracking.btree import TrackingStorage
+  >>> from cybertools.tracking.comment.base import Comment
+
+  >>> comments = TrackingStorage(trackFactory=Comment)
diff --git a/tracking/notify/__init__.py b/tracking/notify/__init__.py
new file mode 100644
index 0000000..4bc90fb
--- /dev/null
+++ b/tracking/notify/__init__.py
@@ -0,0 +1,4 @@
+"""
+$Id$
+"""
+
diff --git a/tracking/notify/base.py b/tracking/notify/base.py
new file mode 100644
index 0000000..bbe9155
--- /dev/null
+++ b/tracking/notify/base.py
@@ -0,0 +1,33 @@
+#
+#  Copyright (c) 2008 Helmut Merz helmutm@cy55.de
+#
+#  This program is free software; you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation; either version 2 of the License, or
+#  (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software
+#  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+
+"""
+Base classes for a notification framework.
+
+$Id$
+"""
+
+from zope.interface import implements
+
+from cybertools.tracking.btree import Track
+from cybertools.tracking.notify.interfaces import INotification
+
+
+class Notification(Track):
+
+    implements(INotification)
diff --git a/tracking/notify/interfaces.py b/tracking/notify/interfaces.py
new file mode 100644
index 0000000..cb98233
--- /dev/null
+++ b/tracking/notify/interfaces.py
@@ -0,0 +1,70 @@
+#
+#  Copyright (c) 2008 Helmut Merz helmutm@cy55.de
+#
+#  This program is free software; you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation; either version 2 of the License, or
+#  (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software
+#  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+
+"""
+Interface definitions for a notification framework.
+
+$Id$
+"""
+
+from zope.interface import Interface, Attribute
+from zope import schema
+
+from cybertools.tracking.interfaces import ITrack
+
+
+class INotification(ITrack):
+    """ A notification carries information necessary to inform a
+        receiver (typically a user or person, but possibly also an other
+        entity) about an event.
+        The object the notification is related to is referenced via the
+        task id attribute; interdependent notifications (i.e. notifications
+        that are triggered by the identical event or have been created
+        by another notification) share the run id; the user name references
+        the user/person that will be notified.
+    """
+
+    type = Attribute('A string (token) that specifies the '
+                'type of the notification.')            # TODO: vocabulary
+    state = Attribute('A string (token) specifying the '
+                'current state of the notification.')   # TODO: vocabulary
+    parent = Attribute('The id of the parent notification, i.e. the '
+                'notification that triggered the creation of this one. '
+                'None if there is not parent notification.')
+    eventType = Attribute('A string (token or title) that specifies the '
+                'type of the event that triggered this notification.')
+    media = Attribute('A string (token) specifying the '
+                'media type that will be used for presenting '
+                'the notification.')                    # TODO: vocabulary
+    timingType = Attribute('A string (token) specifying the '
+                'type of timing that will be used for presenting '
+                'the notification.')                    # TODO: vocabulary
+    timing = Attribute('A list of (typically) time/date values specifying '
+                'the time range for presentation. The possible values and '
+                'their interpretation depend on the timing type.')
+    deliveryInfo = Attribute('Additional information needed for the '
+                'delivery of the notification, e.g. an email address.')
+    actionsPossible = Attribute('A collection of possible actions the receiver '
+                'may take in response to the notification.')
+    actionsTaken = Attribute('One or more actions that have been carried '
+                'out in response to the notification.')
+
+    # media + timingType + timing + deliveryInfo could be combined to
+    # a `deliverySpec` attribute.
+
+    # TODO: Action class (type + instance); vocabulary of action types
diff --git a/tracking/notify/tests.py b/tracking/notify/tests.py
new file mode 100755
index 0000000..33448a6
--- /dev/null
+++ b/tracking/notify/tests.py
@@ -0,0 +1,22 @@
+# $Id$
+
+import unittest, doctest
+from zope.testing.doctestunit import DocFileSuite
+
+
+class Test(unittest.TestCase):
+    "Basic tests for the cybertools.tracking.comment package."
+
+    def testBasics(self):
+        pass
+
+
+def test_suite():
+    flags = doctest.NORMALIZE_WHITESPACE | doctest.ELLIPSIS
+    return unittest.TestSuite((
+                unittest.makeSuite(Test),
+                DocFileSuite('README.txt', optionflags=flags),
+            ))
+
+if __name__ == '__main__':
+    unittest.main(defaultTest='test_suite')