This patch adds PySet_Next(), PySet_Update(), and
PySet_Clear(), three very useful API calls when you're
dealing with concrete PySet objects.

Patch includes tests and documentation.

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Guido, please reject this patch.

I laid-out my reasoning in these two posts:

  http://mail.python.org/pipermail/python-dev/2006-
March/062652.html
  http://mail.python.org/pipermail/python-dev/2006-
March/062737.html

In particular, I've found PySet_Next() to be disasterously 
error-prone (and have had to fix several hard-to-find bugs 
directly attributable this function). There was some need 
for it in the PyDict API because it fetched key and value 
pointers at the same time and allowed direct modification 
of values.  This of course does not apply to sets which do 
not have a value field.

The PySet_Clear() function should be PyObject_Clear() like 
other functions that have tp_slots.  There is no more of a 
case for this function than there would be for PySet_Repr, 
PySet_Len, PySet_IsTrue, or PySet_Print.

The PySet_Update() function gets in the way of my future 
development plans for this module.

None of these functions are necessary.  They simply 
reflect Barry's programming style and views on having fat, 
overly-concrete APIs.

This patch is against my wishes for the module.  As the 
module author and principal maintainer, if I still have a 
say in the module's development, I strongly prefer 
rejection of this patch.

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FWIW, I'm open to PyObject_Clear() for an abstract access 
to the tp_clear slot.  

Also, for Barry's use only, I'm open to a semi-private 
_PySet_Next() that isn't part of the official API but 
gives a Barry-specific hook.  It should not be a public 
function because of all the problems with the approach.

Likewise, _PySet_Update() could be done just for Barry's 
group.  I need to keep PySet_Update() open until Py2.6 
since the API may evolve to allow a variable length 
argument list.

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I'm rejecting this unread, because I'm not ready to jump
into this issue. The discussion should continue without me.

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r43465