Clarify how to add a field to a named tuple.

Doc/library/collections.rst

@@ -538,7 +538,7 @@ faster versions that bypass error-checking and that localize variable access::
 Subclassing is not useful for adding new, stored fields.  Instead, simply
 create a new named tuple type from the :attr:`_fields` attribute::
 
-    >>> Pixel = namedtuple('Pixel', Point._fields + Color._fields)
+    >>> Point3D = namedtuple('Point3D', Point._fields + ('z',))
 
 Default values can be implemented by using :meth:`_replace` to
 customize a prototype instance::

Lib/collections.py

@@ -137,6 +137,9 @@ if __name__ == '__main__':
 
     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())