Function pcurve 

pub fn pcurve(
    surface: &NurbSurface3D,
    original: &Curve2,
    curve: Option<&NurbCurve3D>,
    start: Vector2,
    end: Vector2,
) -> GeopResult<Curve2>

The pcurve on surface of a coedge that runs along curve — or sits at a vertex, without one — from (u, v) start to end. Exact where it can be: on a surface affine in (u, v), a plane, the curve mapped (NurbSurface3D::affine_pcurve); and straight where the coedge it is a copy of ran along an iso-line, original, and its copy does too — the line keeps its u (or v) — as a cylinder’s rim or seam does. Fitted otherwise: a sphere’s meridian cut by a plane is straight, but its inner copy, where the offset plane cuts the offset sphere, is a small circle no straight pcurve follows.