[vtkusers] Hwo to get edges of a vtkpolydata contains triangles?

[pagen_ivan at sohu.com]

Hello:
   I have a vtkpolydata contains triangles. I want to get edges of the triangles. I have search the users' guide but I haven't find something useful to me. 
   Could somebody kindly help me with it? Thank you!
                                                     ivan