Fast sumation functions.
Funcs
func sumKbn[T](x: openArray[T]): T
 Kahan (compensated) summation: O(1) error growth, at the expense of a considerable increase in computational expense. Source Edit
func sumPairs[T](x: openArray[T]): T

Pairwise (cascade) summation of x[i0:i0+n1], with O(log n) error growth (vs O(n) for a simple loop) with negligible performance cost if the base case is large enough.
See, e.g.:
 http://en.wikipedia.org/wiki/Pairwise_summation Higham, Nicholas J. (1993), "The accuracy of floating point summation", SIAM Journal on Scientific Computing 14 (4): 783–799.
In fact, the rootmeansquare error growth, assuming random roundoff errors, is only O(sqrt(log n)), which is nearly indistinguishable from O(1) in practice. See:
 Manfred Tasche and Hansmartin Zeuner, Handbook of AnalyticComputational Methods in Applied Mathematics (2000).