S9 LIB  (erf real)  ==>  real

        (load-from-library "erf.scm")

Gauss error function. Used to compute the integral of the normal
distribution function (NDF).

Using an Abramowitz-Stegun approximation with a maximum error of
3 * 10^-7, i.e. results will start to get flaky beyond the 5th
standard deviation.

(erf 0)  ==>  0.0
