Function normal_sf 

pub fn normal_sf(x: f64) -> f64

Standard normal survival probability P(Z > x).

This is evaluated as ½·erfc(x/√2), not as 1 − Φ(x). The latter loses relative accuracy as soon as Φ(x) approaches one and becomes identically zero for every representable x above roughly 8.3, while the direct complementary form retains the full representable tail.