SF bug #1179957: Missing def'n of equality for set elements
diff --git a/Doc/lib/libsets.tex b/Doc/lib/libsets.tex
index 8085ab1..0cd5e2e 100644
--- a/Doc/lib/libsets.tex
+++ b/Doc/lib/libsets.tex
@@ -30,7 +30,10 @@
abstract class useful for determining whether something is a set:
\code{isinstance(\var{obj}, BaseSet)}.
-The set classes are implemented using dictionaries. As a result, sets
+The set classes are implemented using dictionaries. Accordingly, the
+requirements for set elements are the same as those for dictionary keys;
+namely, that the element defines both \method{__eq__} and \method{__hash__}.
+As a result, sets
cannot contain mutable elements such as lists or dictionaries.
However, they can contain immutable collections such as tuples or
instances of \class{ImmutableSet}. For convenience in implementing