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Toned down note in docs (at Raymond's request) in r65366.
While I'd really like to see an lsum-based version go in instead, I 
think it's too close to the release to make this change right now.
There was also originally some talk of a complex math version. What do 
people think about this? Personally, I suspect that the use cases would 
be few and far between, and anyone who *really* needs a complex high-
precision sum can just apply math.fsum to real and imaginary parts. 
(This is easier now that x.imag and x.real make sense for integers as 
well as floats.)