diff -r b47996cf2ad8 Doc/library/copy.rst
--- a/Doc/library/copy.rst	Fri Mar 20 22:05:33 2015 -0700
+++ b/Doc/library/copy.rst	Sat Mar 21 06:29:34 2015 +0000
@@ -81,7 +81,10 @@
 argument, the memo dictionary.  If the :meth:`__deepcopy__` implementation needs
 to make a deep copy of a component, it should call the :func:`deepcopy` function
 with the component as first argument and the memo dictionary as second argument.
+The :meth:`__deepcopy__` implementation should otherwise consider the memo
+dictionary opaque; only this module may add, remove, or modify the keys of the
+memo dictionary.
 
 
 .. seealso::