Add an index entry for "subclassing immutable types".
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 7304c9c..4dcc96f 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1162,9 +1162,10 @@
Basic customization
-------------------
-
.. method:: object.__new__(cls[, ...])
+ .. index:: pair: subclassing; immutable types
+
Called to create a new instance of class *cls*. :meth:`__new__` is a static
method (special-cased so you need not declare it as such) that takes the class
of which an instance was requested as its first argument. The remaining