Add more entries to the glossary.
Written by Jeff Wheeler for GHOP.
diff --git a/Doc/library/sets.rst b/Doc/library/sets.rst
index f9f8b59..88e442a 100644
--- a/Doc/library/sets.rst
+++ b/Doc/library/sets.rst
@@ -228,9 +228,9 @@
objects are automatically copied to an :class:`ImmutableSet` before being added
as a set element.
-The mechanism is to always add a hashable element, or if it is not hashable, the
-element is checked to see if it has an :meth:`__as_immutable__` method which
-returns an immutable equivalent.
+The mechanism is to always add a :term:`hashable` element, or if it is not
+hashable, the element is checked to see if it has an :meth:`__as_immutable__`
+method which returns an immutable equivalent.
Since :class:`Set` objects have a :meth:`__as_immutable__` method returning an
instance of :class:`ImmutableSet`, it is possible to construct sets of sets.