diff -r 0189b9d2d6bc Modules/gcmodule.c
--- a/Modules/gcmodule.c	Fri May 25 18:42:14 2012 -0400
+++ b/Modules/gcmodule.c	Sat May 26 00:32:19 2012 +0100
@@ -116,6 +116,46 @@
     http://mail.python.org/pipermail/python-dev/2008-June/080579.html
 */
 
+/*
+   NOTE: about untracking of immutable objects.
+   
+   Certain types of container cannot participate in a reference cycle, and
+   so do not need to be tracked by the garbage collector. Untracking these
+   objects reduces the cost of garbage collections. However, determining
+   which objects may be untracked is not free, and the costs must be
+   weighed against the benefits for garbage collection.
+
+   There are two possible strategies for when to untrack a container:
+
+   i) When the container is created.
+   ii) When the container is examined by the garbage collector.
+
+   Tuples containing only immutable objects (integers, strings etc, and
+   recursively, tuples of immutable objects) do not need to be tracked.
+   The interpreter creates a large number of tuples, many of which will
+   not survive until garbage collection. It is therefore not worthwhile
+   to untrack eligible tuples at creation time.
+
+   Instead, all tuples except the empty tuple are tracked when created. 
+   During garbage collection it is determined whether any surviving tuples 
+   can be untracked. A tuple can be untracked if all of its contents are 
+   already not tracked. Tuples are examined for untracking in all garbage 
+   collection cycles. It may take more than one cycle to untrack a tuple.
+
+   Dictionaries containing only immutable objects also do not need to be
+   tracked. Dictionaries are untracked when created. If a tracked item is
+   inserted into a dictionary (either as a key or value), the dictionary
+   becomes tracked. During a full garbage collection (all generations),
+   the collector will untrack any dictionaries whose contents are not
+   tracked.
+
+   The module provides the python function is_tracked(obj), which returns
+   the CURRENT tracking status of the object. Subsequent garbage
+   collections may change the tracking status of the object.
+   
+   Untracking of certain containers was introduced in issue #4688, and 
+   the algorithm was refined in response to issue #14775.
+*/
 
 /* set for debugging information */
 #define DEBUG_STATS             (1<<0) /* print collection statistics */