diff --git a/Lib/typing.py b/Lib/typing.py
index c7fe5dd..1a943ac 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -100,16 +100,16 @@
 
 
 class TypingMeta(type):
-    """Metaclass for every type defined below.
+    """Metaclass for most types defined in typing module
+    (not a part of public API).
 
     This overrides __new__() to require an extra keyword parameter
     '_root', which serves as a guard against naive subclassing of the
     typing classes.  Any legitimate class defined using a metaclass
-    derived from TypingMeta (including internal subclasses created by
-    e.g.  Union[X, Y]) must pass _root=True.
+    derived from TypingMeta must pass _root=True.
 
-    This also defines a dummy constructor (all the work is done in
-    __new__) and a nicer repr().
+    This also defines a dummy constructor (all the work for most typing
+    constructs is done in __new__) and a nicer repr().
     """
 
     _is_protocol = False
@@ -126,8 +126,8 @@
     def _eval_type(self, globalns, localns):
         """Override this in subclasses to interpret forward references.
 
-        For example, Union['C'] is internally stored as
-        Union[_ForwardRef('C')], which should evaluate to _Union[C],
+        For example, List['C'] is internally stored as
+        List[_ForwardRef('C')], which should evaluate to List[C],
         where C is an object found in globalns or localns (searching
         localns first, of course).
         """
@@ -142,7 +142,7 @@
 
 
 class _TypingBase(metaclass=TypingMeta, _root=True):
-    """Indicator of special typing constructs."""
+    """Internal indicator of special typing constructs."""
 
     __slots__ = ()
 
@@ -179,10 +179,10 @@
 
 
 class _FinalTypingBase(_TypingBase, _root=True):
-    """Mix-in class to prevent instantiation.
+    """Internal mix-in class to prevent instantiation.
 
     Prevents instantiation unless _root=True is given in class call.
-    It is used to create pseudo-singleton instances Any, Union, Tuple, etc.
+    It is used to create pseudo-singleton instances Any, Union, Optional, etc.
     """
 
     __slots__ = ()
@@ -198,7 +198,7 @@
 
 
 class _ForwardRef(_TypingBase, _root=True):
-    """Wrapper to hold a forward reference."""
+    """Internal wrapper to hold a forward reference."""
 
     __slots__ = ('__forward_arg__', '__forward_code__',
                  '__forward_evaluated__', '__forward_value__')
@@ -206,11 +206,11 @@
     def __init__(self, arg):
         super().__init__(arg)
         if not isinstance(arg, str):
-            raise TypeError('ForwardRef must be a string -- got %r' % (arg,))
+            raise TypeError('Forward reference must be a string -- got %r' % (arg,))
         try:
             code = compile(arg, '<string>', 'eval')
         except SyntaxError:
-            raise SyntaxError('ForwardRef must be an expression -- got %r' %
+            raise SyntaxError('Forward reference must be an expression -- got %r' %
                               (arg,))
         self.__forward_arg__ = arg
         self.__forward_code__ = code
@@ -336,7 +336,7 @@
 
 
 def _type_check(arg, msg):
-    """Check that the argument is a type, and return it.
+    """Check that the argument is a type, and return it (internal helper).
 
     As a special case, accept None and return type(None) instead.
     Also, _TypeAlias instances (e.g. Match, Pattern) are acceptable.
@@ -363,7 +363,7 @@
 
 
 def _type_repr(obj):
-    """Return the repr() of an object, special-casing types.
+    """Return the repr() of an object, special-casing types (internal helper).
 
     If obj is a type, we return a shorter version than the default
     type.__repr__, based on the module and qualified name, which is
@@ -418,7 +418,7 @@
     as for generic function definitions.  See class Generic for more
     information on generic types.  Generic functions work as follows:
 
-      def repeat(x: T, n: int) -> Sequence[T]:
+      def repeat(x: T, n: int) -> List[T]:
           '''Return a list containing n references to x.'''
           return [x]*n
 
@@ -431,10 +431,7 @@
     that if the arguments are instances of some subclass of str,
     the return type is still plain str.
 
-    At runtime, isinstance(x, T) will raise TypeError.  However,
-    issubclass(C, T) is true for any class C, and issubclass(str, A)
-    and issubclass(bytes, A) are true, and issubclass(int, A) is
-    false.  (TODO: Why is this needed?  This may change.  See #136.)
+    At runtime, isinstance(x, T) and issubclass(C, T) will raise TypeError.
 
     Type variables defined with covariant=True or contravariant=True
     can be used do declare covariant or contravariant generic types.
@@ -509,7 +506,7 @@
 
 
 def _replace_arg(arg, tvars, args):
-    """ A helper fuunction: replace arg if it is a type variable
+    """An internal helper function: replace arg if it is a type variable
     found in tvars with corresponding substitution from args or
     with corresponding substitution sub-tree if arg is a generic type.
     """
@@ -526,9 +523,15 @@
 
 
 def _subs_tree(cls, tvars=None, args=None):
-    """ Calculate substitution tree for generic cls after
-    replacing its type parameters with substitutions in tvars -> args (if any).
-    Repeat the same cyclicaly following __origin__'s.
+    """An internal helper function: calculate substitution tree
+    for generic cls after replacing its type parameters with
+    substitutions in tvars -> args (if any).
+    Repeat the same following __origin__'s.
+
+    Return a list of arguments with all possible substitutions
+    performed. Arguments that are generic classes themselves are represented
+    as tuples (so that no new classes are created by this function).
+    For example: _subs_tree(List[Tuple[int, T]][str]) == [(Tuple, int, str)]
     """
 
     if cls.__origin__ is None:
@@ -553,7 +556,7 @@
 
 
 def _remove_dups_flatten(parameters):
-    """ A helper for Union creation and substitution: flatten Union's
+    """An internal helper for Union creation and substitution: flatten Union's
     among parameters, then remove duplicates and strict subclasses.
     """
 
@@ -594,7 +597,7 @@
 
 
 def _check_generic(cls, parameters):
-    # Check correct count for parameters of a generic cls.
+    # Check correct count for parameters of a generic cls (internal helper).
     if not cls.__parameters__:
         raise TypeError("%s is not a generic class" % repr(cls))
     alen = len(parameters)
@@ -608,7 +611,7 @@
 
 
 def _tp_cache(func):
-    """ Caching for __getitem__ of generic types with a fallback to
+    """Internal wrapper caching __getitem__ of generic types with a fallback to
     original function for non-hashable arguments.
     """
 
@@ -788,7 +791,7 @@
 
 
 def _gorg(a):
-    """Return the farthest origin of a generic class."""
+    """Return the farthest origin of a generic class (internal helper)."""
     assert isinstance(a, GenericMeta)
     while a.__origin__ is not None:
         a = a.__origin__
@@ -796,10 +799,10 @@
 
 
 def _geqv(a, b):
-    """Return whether two generic classes are equivalent.
+    """Return whether two generic classes are equivalent (internal helper).
 
     The intention is to consider generic class X and any of its
-    parameterized forms (X[T], X[int], etc.)  as equivalent.
+    parameterized forms (X[T], X[int], etc.) as equivalent.
 
     However, X is not equivalent to a subclass of X.
 
@@ -825,6 +828,7 @@
 
 
 def _valid_for_check(cls):
+    """An internal helper to prohibit isinstance([1], List[str]) etc."""
     if cls is Generic:
         raise TypeError("Class %r cannot be used with class "
                         "or instance checks" % cls)
@@ -1083,8 +1087,8 @@
 class Generic(metaclass=GenericMeta):
     """Abstract base class for generic types.
 
-    A generic type is typically declared by inheriting from an
-    instantiation of this class with one or more type variables.
+    A generic type is typically declared by inheriting from
+    this class parameterized with one or more type variables.
     For example, a generic mapping type might be defined as::
 
       class Mapping(Generic[KT, VT]):
@@ -1111,18 +1115,18 @@
 
 
 class _TypingEmpty:
-    """Placeholder for () or []. Used by TupleMeta and CallableMeta
-    to allow empy list/tuple in specific places, without allowing them
+    """Internal placeholder for () or []. Used by TupleMeta and CallableMeta
+    to allow empty list/tuple in specific places, without allowing them
     to sneak in where prohibited.
     """
 
 
 class _TypingEllipsis:
-    """Ditto for ..."""
+    """Internal placeholder for ... (ellipsis)."""
 
 
 class TupleMeta(GenericMeta):
-    """Metaclass for Tuple"""
+    """Metaclass for Tuple (internal)."""
 
     @_tp_cache
     def __getitem__(self, parameters):
@@ -1175,7 +1179,7 @@
 
 
 class CallableMeta(GenericMeta):
-    """ Metaclass for Callable."""
+    """Metaclass for Callable (internal)."""
 
     def __repr__(self):
         if self.__origin__ is None:
@@ -1199,7 +1203,7 @@
                 '[[%s], %s]' % (', '.join(arg_list[:-1]), arg_list[-1]))
 
     def __getitem__(self, parameters):
-        """ A thin wrapper around __getitem_inner__ to provide the latter
+        """A thin wrapper around __getitem_inner__ to provide the latter
         with hashable arguments to improve speed.
         """
 
@@ -1236,7 +1240,7 @@
 
     The subscription syntax must always be used with exactly two
     values: the argument list and the return type.  The argument list
-    must be a list of types; the return type must be a single type.
+    must be a list of types or ellipsis; the return type must be a single type.
 
     There is no syntax to indicate optional or keyword arguments,
     such function types are rarely used as callback types.
@@ -1565,7 +1569,7 @@
 class _Protocol(metaclass=_ProtocolMeta):
     """Internal base class for protocol classes.
 
-    This implements a simple-minded structural isinstance check
+    This implements a simple-minded structural issubclass check
     (similar but more general than the one-offs in collections.abc
     such as Hashable).
     """