Function coons_bilinear 

pub fn coons_bilinear(
    boundaries: [NurbsCurve; 4],
) -> Result<NurbsSurface, CoonsRefusal>

The bilinearly blended Coons patch through four boundary curves given in cyclic order.

See the module documentation for the contract. Use coons_bilinear_reported when the measurements matter — which reversals were applied, what the corners actually measured, whether the boundaries came back bit for bit.