Function curve_curve_overlaps_and_crossings 

pub fn curve_curve_overlaps_and_crossings<const D: usize, const C: usize>(
    a: &NurbCurve<D>,
    b: &NurbCurve<D>,
    reach: Scalar,
    max_nodes: usize,
    min_subdivision_size: Scalar,
) -> GeopResult<OverlapsAndCrossings>
where
    NurbCurve<D>: ParameterRefinable<C>,

Overlaps of a with b, and the isolated crossings away from them (the clipping search run on each stretch of a between overlaps).

With reach zero an overlap is a stretch lying on b. Above zero it is one running within reach of b: for a caller that works to a resolution of its own — a drawing to what a sheet shows — two curves that close are one line, and are taken as one. Without it the clipping search has to cover such a stretch with pairs of pieces no larger than min_subdivision_size, which no node budget does for a long one.