Implement, test and document "key in dict" and "key not in dict".
I know some people don't like this -- if it's really controversial,
I'll take it out again. (If it's only Alex Martelli who doesn't like
it, that doesn't count as "real controversial" though. :-)
That's why this is a separate checkin from the iterators stuff I'm
about to check in next.
diff --git a/Doc/ref/ref5.tex b/Doc/ref/ref5.tex
index 5663daa..ec0c6b1 100644
--- a/Doc/ref/ref5.tex
+++ b/Doc/ref/ref5.tex
@@ -768,7 +768,9 @@
The set membership test has traditionally been bound to sequences; an
object is a member of a set if the set is a sequence and contains an
element equal to that object. However, it is possible for an object
-to support membership tests without being a sequence.
+to support membership tests without being a sequence. In particular,
+dictionaries support memership testing as a nicer way of spelling
+\code{\var{key} in \var{dict}}; other mapping types may follow suit.
For the list and tuple types, \code{\var{x} in \var{y}} is true if and
only if there exists an index \var{i} such that