Add another usage note for collections.Set
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index 1e32f99..493e22a 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -58,7 +58,7 @@
``insert``, ``remove``, and ``__iadd__``
and ``__len__``
-:class:`Set` \(1) \(2) :class:`Sized`, ``__len__``, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``,
+:class:`Set` :class:`Sized`, ``__len__``, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``,
:class:`Iterable`, ``__iter__``, and ``__gt__``, ``__ge__``, ``__and__``, ``__or__``
:class:`Container` ``__contains__`` ``__sub__``, ``__xor__``, and ``isdisjoint``
@@ -118,6 +118,13 @@
semantics are fixed), redefine :meth:`__le__` and
then the other operations will automatically follow suit.
+(3)
+ The :class:`Set` mixin provides a :meth:`_hash` method to compute a hash value
+ for the set; however, :meth:`__hash__` is not defined because not all sets
+ are hashable or immutable. To add set hashabilty using mixins,
+ inherit from both :meth:`Set` and :meth:`Hashable`, then define
+ ``__hash__ = Set._hash``.
+
(For more about ABCs, see the :mod:`abc` module and :pep:`3119`.)