diff --git a/pep-0575.rst b/pep-0575.rst
index da9d99c91..4576a29a4 100644
--- a/pep-0575.rst
+++ b/pep-0575.rst
@@ -95,6 +95,8 @@ with some differences:
    replacing ``method_descriptor``.
    Also ``__qualname__`` will use ``__objclass__`` as namespace
    (instead of ``m_self`` before).
+   For methods of extension types, it is automatically assigned as the defining class
+   (``__class__`` in plain Python).
 
 #. Argument Clinic [#clinic]_ is not supported.
 
@@ -139,7 +141,7 @@ These are the relevant C structures::
         PyObject_HEAD
         PyCFunctionDef *m_ml;     /* Description of the C function to call */
         PyObject *m_self;         /* __self__: anything, can be NULL; readonly */
-        PyObject *m_module;       /* __module__: anything */
+        PyObject *m_module;       /* __module__: anything (typically str) */
         PyTypeObject *m_objclass; /* __objclass__: type or NULL; readonly */
         PyObject *m_weakreflist;  /* List of weak references */
     } PyBaseFunctionObject;
@@ -224,6 +226,11 @@ And ``__defaults__`` is not required to be used for calling the function.
 Apart from adding these extra attributes,
 ``defined_function`` behaves exactly the same as ``base_function``.
 
+**TODO**: maybe find a better name for ``defined_function``.
+Other proposals: ``builtout_function`` (a function that is better built out; pun on builtin),
+``generic_function`` (original proposal but conflicts with ``functools.singledispatch`` generic functions),
+``user_function`` (defined by the user as opposed to CPython).
+
 function
 --------
 
@@ -808,6 +815,64 @@ are deprecated and no longer used by CPython itself.
 For now, they are kept for backwards compatibility.
 
 
+Two-phase Implementation
+========================
+
+**TODO**: this section is optional. When accepting this PEP, it should
+be decided whether to apply this two-phase implementation or not.
+
+As mentioned above, the `changes to types and inspect`_ can break some
+existing code.
+In order to further minimize breakage, this PEP could be implemented
+in two phases.
+
+Phase one: duplicate classes
+----------------------------
+
+Implement this PEP but duplicate the classes ``bound_method``
+and ``builtin_function``.
+Add a new class ``builtin_method`` which is an exact copy of ``builtin_function``
+add a new class ``bound_builtin_method`` which is an exact copy
+of ``bound_method`` (in both cases, only the name would differ).
+
+The class ``builtin_method`` will be used for unbound methods
+of extension types.
+It should be seen as continuation of the existing class
+``method-descriptor``.
+This ensures 100% backwards compatibility for these objects
+(except for added attributes and maybe the ``repr()``).
+
+The same would be done for bound methods of extension types:
+these will be instances of ``bound_builtin_method``.
+This ensures full backwards compatibility, except for code
+assuming that ``types.BuiltinFunctionType`` is the same as ``types.BuiltinMethodType``.
+
+For ``inspect``, we keep but deprecate the functions
+``isbuiltin``, ``ismethod`` and ``ismethoddescriptor``.
+To replace these, new functions ``isbuiltinfunction``, ``isboundmethod``
+and ``isgetdescriptor`` (other possible names: ``isreaddescriptor`` or ``isdescriptor``)
+are added.
+The function ``isbuiltinfunction`` checks for instances of ``builtin_function``
+and ``builtin_method``.
+``isboundmethod`` checks for both ``bound_method`` and ``bound_builtin_method``.
+And ``isgetdescriptor`` checks for non-data descriptors
+which are not instances of ``base_function``.
+
+Since showing an actual ``DeprecationWarning`` would affect a lot
+of correctly-functioning code,
+the deprecations would only appear in the documentation.
+Another reason is that it is hard to show warnings for calling ``isinstance(x, t)``
+(but it could be done using ``__instancecheck__`` hacking)
+and impossible for ``type(x) is t``.
+
+Phase two
+---------
+
+Phase two is what is actually described in the rest of this PEP:
+the duplicate classes would be merged and the ``inspect`` functions
+adjusted accordingly.
+
+
 Reference Implementation
 ========================