Function tangent_groups 

pub fn tangent_groups(
    pieces: &[(&NurbSurface3D, (Scalar, Scalar))],
) -> GeopResult<Vec<Vec<usize>>>

pieces, each a surface and where a point is on it, grouped into the smooth surfaces they are pieces of there: those whose normals there could be parallel. The groups as indices into pieces, in order.

Decided once, at the point, not at the foot points a search moves through: each piece clamps those to its own patch, their normals there differ a little, and nearly equal conditions leave no step to take.