More extensive comment on __exit__ handling, per Guido's request.

Lib/contextlib.py

@@ -34,6 +34,13 @@ class GeneratorContextManager(object):
             except StopIteration:
                 return True
             except:
+                # only re-raise if it's *not* the exception that was
+                # passed to throw(), because __exit__() must not raise
+                # an exception unless __exit__() itself failed.  But throw()
+                # has to raise the exception to signal propagation, so this
+                # fixes the impedance mismatch between the throw() protocol
+                # and the __exit__() protocol.
+                #
                 if sys.exc_info()[1] is not value:
                     raise