PEP 342 implementation.  Per Guido's comments, the generator throw()
method still needs to support string exceptions, and allow None for the
third argument.  Documentation updates are needed, too.
diff --git a/Lib/test/test_genexps.py b/Lib/test/test_genexps.py
index 04694f8..7c6fe4a 100644
--- a/Lib/test/test_genexps.py
+++ b/Lib/test/test_genexps.py
@@ -130,7 +130,7 @@
     >>> (y for y in (1,2)) += 10
     Traceback (most recent call last):
        ...
-    SyntaxError: augmented assign to tuple literal or generator expression not possible
+    SyntaxError: augmented assign to tuple literal, yield, or generator expression not possible