diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 971c06e..36fc575 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -59,13 +59,16 @@
 they may be garbage-collected.  An implementation is allowed to postpone garbage
 collection or omit it altogether --- it is a matter of implementation quality
 how garbage collection is implemented, as long as no objects are collected that
-are still reachable.  (Implementation note: CPython currently uses a
-reference-counting scheme with (optional) delayed detection of cyclically linked
-garbage, which collects most objects as soon as they become unreachable, but is
-not guaranteed to collect garbage containing circular references.  See the
-documentation of the :mod:`gc` module for information on controlling the
-collection of cyclic garbage.  Other implementations act differently and CPython
-may change.)
+are still reachable.
+
+.. impl-detail::
+
+   CPython currently uses a reference-counting scheme with (optional) delayed
+   detection of cyclically linked garbage, which collects most objects as soon
+   as they become unreachable, but is not guaranteed to collect garbage
+   containing circular references.  See the documentation of the :mod:`gc`
+   module for information on controlling the collection of cyclic garbage.
+   Other implementations act differently and CPython may change.
 
 Note that the use of the implementation's tracing or debugging facilities may
 keep objects alive that would normally be collectable. Also note that catching
@@ -1469,15 +1472,15 @@
   *__slots__*; otherwise, the class attribute would overwrite the descriptor
   assignment.
 
+* The action of a *__slots__* declaration is limited to the class where it is
+  defined.  As a result, subclasses will have a *__dict__* unless they also define
+  *__slots__* (which must only contain names of any *additional* slots).
+
 * If a class defines a slot also defined in a base class, the instance variable
   defined by the base class slot is inaccessible (except by retrieving its
   descriptor directly from the base class). This renders the meaning of the
   program undefined.  In the future, a check may be added to prevent this.
 
-* The action of a *__slots__* declaration is limited to the class where it is
-  defined.  As a result, subclasses will have a *__dict__* unless they also define
-  *__slots__*.
-
 * Nonempty *__slots__* does not work for classes derived from "variable-length"
   built-in types such as :class:`int`, :class:`str` and :class:`tuple`.
 
@@ -1714,12 +1717,16 @@
 supply the following special method with a more efficient implementation, which
 also does not require the object be a sequence.
 
-
 .. method:: object.__contains__(self, item)
 
-   Called to implement membership test operators.  Should return true if *item* is
-   in *self*, false otherwise.  For mapping objects, this should consider the keys
-   of the mapping rather than the values or the key-item pairs.
+   Called to implement membership test operators.  Should return true if *item*
+   is in *self*, false otherwise.  For mapping objects, this should consider the
+   keys of the mapping rather than the values or the key-item pairs.
+
+   For objects that don't define :meth:`__contains__`, the membership test first
+   tries iteration via :meth:`__iter__`, then the old sequence iteration
+   protocol via :meth:`__getitem__`, see :ref:`this section in the language
+   reference <membership-test-details>`.
 
 
 .. _numeric-types:
diff --git a/Doc/reference/executionmodel.rst b/Doc/reference/executionmodel.rst
index 68ee654..90791d2 100644
--- a/Doc/reference/executionmodel.rst
+++ b/Doc/reference/executionmodel.rst
@@ -129,7 +129,7 @@
 itself.  ``__builtins__`` can be set to a user-created dictionary to create a
 weak form of restricted execution.
 
-.. note::
+.. impl-detail::
 
    Users should not touch ``__builtins__``; it is strictly an implementation
    detail.  Users wanting to override values in the built-in namespace should
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index cdb802a..d074ebb 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -639,13 +639,13 @@
 raised.  Otherwise, the list of filled slots is used as the argument list for
 the call.
 
-.. note::
+.. impl-detail::
 
-   An implementation may provide built-in functions whose positional parameters do
-   not have names, even if they are 'named' for the purpose of documentation, and
-   which therefore cannot be supplied by keyword.  In CPython, this is the case for
-   functions implemented in C that use :cfunc:`PyArg_ParseTuple` to parse their
-   arguments.
+   An implementation may provide built-in functions whose positional parameters
+   do not have names, even if they are 'named' for the purpose of documentation,
+   and which therefore cannot be supplied by keyword.  In CPython, this is the
+   case for functions implemented in C that use :cfunc:`PyArg_ParseTuple` to
+   parse their arguments.
 
 If there are more positional arguments than there are formal parameter slots, a
 :exc:`TypeError` exception is raised, unless a formal parameter using the syntax
@@ -1053,6 +1053,8 @@
 supported cross-type comparisons and unsupported comparisons.  For example,
 ``Decimal(2) == 2`` and `2 == float(2)`` but ``Decimal(2) != float(2)``.
 
+.. _membership-test-details:
+
 The operators :keyword:`in` and :keyword:`not in` test for membership.  ``x in
 s`` evaluates to true if *x* is a member of *s*, and false otherwise.  ``x not
 in s`` returns the negation of ``x in s``.  All built-in sequences and set types
@@ -1069,7 +1071,12 @@
 For user-defined classes which define the :meth:`__contains__` method, ``x in
 y`` is true if and only if ``y.__contains__(x)`` is true.
 
-For user-defined classes which do not define :meth:`__contains__` and do define
+For user-defined classes which do not define :meth:`__contains__` but do define
+:meth:`__iter__`, ``x in y`` is true if some value ``z`` with ``x == z`` is
+produced while iterating over ``y``.  If an exception is raised during the
+iteration, it is as if :keyword:`in` raised that exception.
+
+Lastly, the old-style iteration protocol is tried: if a class defines
 :meth:`__getitem__`, ``x in y`` is true if and only if there is a non-negative
 integer index *i* such that ``x == y[i]``, and all lower integer indices do not
 raise :exc:`IndexError` exception.  (If any other exception is raised, it is as
diff --git a/Doc/reference/simple_stmts.rst b/Doc/reference/simple_stmts.rst
index a8ec495..b1be34a 100644
--- a/Doc/reference/simple_stmts.rst
+++ b/Doc/reference/simple_stmts.rst
@@ -236,9 +236,11 @@
   from the length of the assigned sequence, thus changing the length of the
   target sequence, if the object allows it.
 
-(In the current implementation, the syntax for targets is taken to be the same
-as for expressions, and invalid syntax is rejected during the code generation
-phase, causing less detailed error messages.)
+.. impl-detail::
+
+   In the current implementation, the syntax for targets is taken to be the same
+   as for expressions, and invalid syntax is rejected during the code generation
+   phase, causing less detailed error messages.
 
 WARNING: Although the definition of assignment implies that overlaps between the
 left-hand side and the right-hand side are 'safe' (for example ``a, b = b, a``
@@ -937,9 +939,11 @@
 parameters or in a :keyword:`for` loop control target, :keyword:`class`
 definition, function definition, or :keyword:`import` statement.
 
-(The current implementation does not enforce the latter two restrictions, but
-programs should not abuse this freedom, as future implementations may enforce
-them or silently change the meaning of the program.)
+.. impl-detail::
+
+   The current implementation does not enforce the latter two restrictions, but
+   programs should not abuse this freedom, as future implementations may enforce
+   them or silently change the meaning of the program.
 
 .. index::
    builtin: exec