Recorded merge of revisions 74228 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/trunk

Applied in r74226.

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  r74228 | raymond.hettinger | 2009-07-27 16:32:04 -0400 (Mon, 27 Jul 2009) | 1 line

  Issue 6573: Fix set.union() for cases where self is in the argument chain.
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