diff --git a/Lib/collections.py b/Lib/collections.py
index 47b0397..267c39f 100644
--- a/Lib/collections.py
+++ b/Lib/collections.py
@@ -137,6 +137,9 @@
 
     print Point(11, 22)._replace(x=100)
 
+    Point3D = namedtuple('Point3D', Point._fields + ('z',))
+    print Point3D.__doc__
+
     import doctest
     TestResults = namedtuple('TestResults', 'failed attempted')
     print TestResults(*doctest.testmod())