Function is_tail 

pub fn is_tail(
    model: &Model,
    (surface_id, surface): (SurfaceId, &SurfaceGeometry),
    out: &Shape,
    back: &Shape,
    uses: &FastMap<SameKey, usize>,
    tol: Tolerances,
) -> OgeomResult<bool>

Whether out, then back, walked in that order by one wire of the face on surface, are a tail. uses counts how many times the faces of the shape walk each edge as placed (placed_edge_uses). See the module documentation.

Errors

As out_and_back.