Constant RELATIVE_ROUNDING 

pub const RELATIVE_ROUNDING: f64 = _; // 1.7763568394002505E-15f64

Relative rounding slack: a magnitude at or below this fraction of its natural scale is rounding noise, not a value. Eight ulps — what a handful of multiplications and one trigonometric evaluation leave behind, and orders of magnitude below any model tolerance. It is not a geometric tolerance and never decides whether two things are the same; it decides whether a computed quantity is zero in floating point: the radius of a sphere’s parallel at the pole, cos(π/2) evaluated in f64, is 6e-17, not 0.