More accurate summations [was Re: This math scares me]

[Edward Jason Riedy]

And I mistakenly blather:
 - 
 - The accumulated error in s from floating-point calculations is at 
 - most one unit in the last place of each summand x, [...]

Geez, I can't get anything right.  It's not one ulp, but
rather the distance from 1.0 to the next higher floating-
point number (either epsilon or 2*epsilon, depending on
whose definition you use).  But self.c still gives a bound
on the total rounding error.

Jason
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