Function erf 

pub fn erf(x: f64) -> f64

Error function: erf(x) = 2/sqrt(pi) * integral_0^x exp(-t^2) dt.

Odd (erf(-x) == -erf(x)) and bounded in [-1, 1]. erf(NAN) is NAN; erf(+-INFINITY) is +-1.0 exactly.