Topology Explorer

Hello,

I have a question concerning the functionality of the Topology Explorer TopExp_Explorer.
I read an IGES-file (B-Spline surfaces) to one shape (TopoDS_Shape). Now I want to find shared edges with the help of the example from the modeling data users guide (page 49), but none of the edges seems to be shared. I think for this sharing information I have to build up the shape from the ground, that means edges from vertexes, then wires, faces, shells.
Is this correct? Are there any methods to explore the topology of an IGES-file?

Thanks for any comments,
Gernot Knieling