Function chain_certifies 

pub fn chain_certifies(chain: &Chain2) -> bool

Whether crate::curve::flatten2 certifies a chain’s chords: every intrinsic piece’s curvature is bounded and every parametric piece’s curve is a line, a circle, an ellipse or a B-spline with no corner anywhere in its domain (#232, #438).

Conservative: a corner outside the stretch a piece reads still answers false, since locating the stretch needs the arc-length inverse.