Document that __cmp__() is not defined for sets.
Note, that list.sort() is undefined for lists of sets.
Add the ... prompt to the example so it runs in doctest.
diff --git a/Doc/lib/libsets.tex b/Doc/lib/libsets.tex
index 046463e..646e205 100644
--- a/Doc/lib/libsets.tex
+++ b/Doc/lib/libsets.tex
@@ -110,6 +110,15 @@
A set is greater than another set if and only if the first set is a proper
superset of the second set (is a superset, but is not equal).
+The subset and equality comparisons do not generalize to a complete
+ordering function. For example, any two disjoint sets are not equal and
+are not subsets of each other, so \emph{none} of the following are true:
+\code{\var{a}<\var{b}}, \code{\var{a}==\var{b}}, or \code{\var{a}>\var{b}}.
+Accordingly, sets do not implement the \method{__cmp__} method.
+
+Since sets only define partial ordering (subset relationships), the output
+of the \method{list.sort()} method is undefined for lists of sets.
+
The following table lists operations available in \class{ImmutableSet}
but not found in \class{Set}:
@@ -175,9 +184,9 @@
>>> employees.issuperset(engineers)
True
>>> for group in [engineers, programmers, management, employees]:
- group.discard('Susan') # unconditionally remove element
- print group
-
+... group.discard('Susan') # unconditionally remove element
+... print group
+...
Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
Set(['Janice', 'Jack', 'Sam'])
Set(['Jane', 'Zack', 'Jack'])