Error function related functions.
The derivative of erf is x -> 2 / sqrt pi * exp (x^2), and this uniquely determines erf by erf 0 = 0.
Minimal complete definition is erfc or normcdf.
erfc x = 1 - erf x
erfcx x = exp (x*x) * erfc x
normcdf x = erfc(-x sqrt 2) 2