Function erfcx_nonnegative 

pub fn erfcx_nonnegative(x: f64) -> f64

Scaled complementary error function erfcx(x) = exp(x²) · erfc(x), specialized to x ≥ 0. Returns 1.0 for x ≤ 0 and 0.0 for x = +∞. For 0 < x < 26 uses the direct exp(x²)·erfc(x) form; beyond that the (otherwise overflowing) exp(x²) is replaced by a 4-term asymptotic expansion (1/(x√π))·(1 − 1/(2x²) + 3/(4x⁴) − …), keeping relative accuracy near machine epsilon. The non-negative restriction lets the caller skip the reflection identity.