Function stirlerr_terms 

pub const fn stirlerr_terms(precision: PrecisionPolicy) -> usize

Terms of the 1/n^2 series in stirlerr by tier, at n >= STIRLERR_MIN. The dropped term bounds the absolute error, which is the relative error of whatever density it feeds: 9 terms is 1e-18, 6 is 2.5e-15 (a measured 5 ulp at n = 9, so Average keeps all 9, the three FMAs being nothing), 5 is 1e-13, and the single 1/(12n) term is 3.8e-6. Worst forgoes the series and keeps just that.