Add another named tuple subclassing example.
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index d7d1083..564c45b 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -527,6 +527,14 @@
Point(x=2.000, y=5.000, hypot=5.385)
Point(x=1.286, y=6.000, hypot=6.136)
+Another use for subclassing is to replace performance critcal methods with
+faster versions that bypass error-checking and localize variable access:
+
+ >>> class Point(namedtuple('Point', 'x y')):
+ _make = classmethod(tuple.__new__)
+ def _replace(self, _map=map, **kwds):
+ return self._make(_map(kwds.pop, ('x', 'y'), self))
+
Default values can be implemented by starting with a prototype instance
and customizing it with :meth:`_replace`: