Make "hashable" a glossary entry and clarify docs on __cmp__, __eq__ and __hash__.
I hope the concept of hashability is better understandable now.
Thanks to Tim Hatch for pointing out the flaws here.
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 488f090..706d0f1 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -276,7 +276,7 @@
.. index:: pair: immutable; object
Restrictions on the types of the key values are listed earlier in section
-:ref:`types`. (To summarize, the key type should be hashable, which excludes
+:ref:`types`. (To summarize, the key type should be :term:`hashable`, which excludes
all mutable objects.) Clashes between duplicate keys are not detected; the last
datum (textually rightmost in the display) stored for a given key value
prevails.