D.6.1 gslevu calling conventions

The input to either of these functions is a list of real numbers represented as explained in math.

The result is a pair (s,e) holding an estimate of the sum, s, and an estimate of the error in the sum, e, each being a real number.

Both functions compute the same sum, s, but the utrunc function is faster and more memory efficient, using a less trustworthy method of estimating the error.