Function surface_iso_u_curve 

pub fn surface_iso_u_curve(
    surface: &SurfaceGeometry,
    at: f64,
    tol: Tolerances,
) -> Option<Curve>

The surface’s u = at iso-curve, parameterized by v exactly.

A ruling on a cylinder, a tube circle on a torus: the curve a seam runs along. None where no closed form exists. Shared by the STEP reader’s seam synthesis and the healer’s ring re-anchoring, so the two cannot disagree about what a seam is.