Function surface_curve_sweep_within 

pub fn surface_curve_sweep_within(
    rings: &Rings,
    sample: impl FnMut(Scalar) -> GeomResult<(SampledPath, Vec<Vec3>)>,
    budget: Scalar,
) -> GeomResult<TriMesh>

surface_curve_sweep whose walls stay within budget of the surface its rings trace along a smooth directrix (#231).

sample(b) returns the directrix sampled to chord budget b and the reference surface’s normal at each sample; see fixed_reference_sweep_within.