Add some cross-references to the docs.  Simplify the python code equivalent for zip().  Supply an optional argument for the nth() recipe.
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 491cb18..32ad792 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -615,9 +615,9 @@
        "Return function(0), function(1), ..."
        return map(function, count(start))
 
-   def nth(iterable, n):
-       "Returns the nth item or None"
-       return next(islice(iterable, n, None), None)
+   def nth(iterable, n, default=None):
+       "Returns the nth item or a default value"
+       return next(islice(iterable, n, None), default)
 
    def quantify(iterable, pred=bool):
        "Count how many times the predicate is true"