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Thank you for the superb contribution.

It passes all of my tests without exception (see the new
file, test_genexps).  I do not see any issues with the coding.

Am accepting and applying the patch except for the
documentation.  

Please update the documentation part of the patch and I will
go through and clarify where necessary.  Also, I will update
the PEP to reflect Guido's rationale for the design
decisions on binding behavior.