pub fn normal_logcdf(x: f64) -> f64Expand description
Numerically stable ln Φ(x) for the standard normal CDF. For x ≥ 0
computes ln(Φ(x)) directly with a small floor against underflow; for
x < 0 rewrites
ln Φ(x) = −u² + ln(½·erfcx(u)), u = −x/√2,
which preserves digits all the way into the deep left tail (no
ln(0)). Returns ±∞ and NaN at the corresponding inputs.