Function station_frame_is_exact2 

pub fn station_frame_is_exact2(curve: &Curve2) -> bool

Whether a station frame on this 2D curve is exact (#264): its point and axes at the station’s distance itself, in closed form, rounding aside, with no quadrature or root find behind them.

Only a line: on every other family the distance is read by the arc-length inverse (ARC_LENGTH_TOLERANCE, an estimate) or the point by quadrature (an intrinsic curve, a chain), so the frame is not exact and a placement in it must not be reported as exact.