Function exact_pcurve_over 

pub fn exact_pcurve_over(
    curve: &Curve,
    range: (f64, f64),
    surface: &SurfaceGeometry,
    tol: Tolerances,
) -> Option<PlanarCurve>

As exact_pcurve_of, with the parameter range the caller actually uses.

A curve’s chart image can depend on which part of the curve is meant: a ruling on a cone crosses the apex, and its angle on the far nappe is half a turn from its angle on the near one. The curve’s own domain may span both (an imported line’s usually does), so a caller that knows its edge’s range must say so, or the exact projection may answer for the wrong side.