@@ -2037,12 +2037,15 @@ Basic customization
    .. index:: single: __len__() (mapping object method)
 
    Called to implement truth value testing and the built-in operation
-   ``bool()``; should return ``False`` or ``True``.  When this method is not
+   :func:`bool`; should return ``False`` or ``True``.  When this method is not
    defined, :meth:`~object.__len__` is called, if it is defined, and the object is
    considered true if its result is nonzero.  If a class defines neither
    :meth:`!__len__` nor :meth:`!__bool__`, all its instances are considered
    true.
 
+   Two successive calls to :meth:`!__bool__` on the same object must
+   return same value.
+
 
 .. _attribute-access:
 
@@ -2947,6 +2950,9 @@ through the object's keys; for sequences, it should iterate through the values.
    :meth:`~object.__bool__` method and whose :meth:`!__len__` method returns zero is
    considered to be false in a Boolean context.
 
+   Two successive calls to :meth:`!__len__` on the same object must
+   return same value.
+
    .. impl-detail::
 
       In CPython, the length is required to be at most :data:`sys.maxsize`.