Function recognize_swept_envelope 

pub fn recognize_swept_envelope(
    profile: &[NurbsCurve],
    path: &SweepPath,
    placement_mode: SectionPlacement,
) -> Result<Result<SweptEnvelope, String>, String>

Recognize the one configuration the envelope lane builds, or say — in words a refusal can quote — which half of it this sweep is not.

Ok(Ok(plan)) is the configuration; Ok(Err(reason)) is a measured reason it is not. A path that is not tight at all reads Err too: this lane only ever speaks past ρκ = 1, and below it the skinning lanes own the shape.