Function cmp_along 

pub fn cmp_along(a: &ImplicitPoint, b: &ImplicitPoint, u: [f64; 3]) -> Sign

Exact sign of (a − b)·u — order two points along direction u, over ANY Explicit/Lpi/Tpi mix (generalises lpi_compare_along). With a=λa/da, b=λb/db: (a−b)·u = ((λa·u)·db − (λb·u)·da)/(da·db).