Function boundary_curve 

pub fn boundary_curve(
    patch: &NurbSurface<4>,
    u_fixed: bool,
    first: bool,
) -> Option<NurbCurve<4>>

The boundary of patch at the start (first) or end of its u domain (u_fixed) or v domain, as a curve along the other parameter — the first/last control row, which is the surface there when the knot vector is clamped at that end. None if it isn’t.