Function piece_frame_bounds 

pub fn piece_frame_bounds(
    piece: &StationPiece<'_>,
    from: f64,
    to: f64,
    reading: FrameReading,
) -> Option<FrameLawBounds>

Certified bounds on the derivatives, in station distance, of the section frame piece presents over [from, to] (distances into the piece, in the composite’s direction), read as reading; see the module documentation.

The span must lie inside the piece and cross no seam of its curve (StationPiece’s seams, a polyline’s vertices, a piece of an elevation, cant or pivot law); a caller splits there.

None where the frame law is not certified: an ellipse, a B-spline, an intrinsic curve (only FrameLaw certifies those, #361), a chain, an offset, a skewed circle, an elevated or banked curve whose plan is not a line or a circle or whose profile is not of closed-form pieces, a span across a seam, a tangent too steep to bound its horizontal part, a roll reaching a quarter turn, and a plan-measured piece under a placement that tilts +Z.