Rev 5
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- Made throwing GeneratorExit into a delegating generator behave the
  same way as closing it.

Rev 4
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- Converted for Python 3.1.2

Rev 3
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- Fixed a refcount bug in gen_iternext().

Rev 2
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- Fixed bug in generator return value handling.
- Fixed NULL return without exception set.
- Allowed 'yield from' directly as a function argument.