[vtkusers] How to insert edges?

[Petru Pau]

After weeks of failures and bad results, I have discovered the reason:
in the huge vtkPolyData that I read as input, some quads do not have
coplanar vertrices. (Side question: Can it be that vtk avoids making 
planarity
tests for quads and polygons?)

The most immediate solution is to insert a few new edges: the diagonals of
these quads.

I tried to do that and failed. First, simply inserting next cell does 
not work,
if the list of lines of the poly data is null. Second, if I take the 
lines of
the poly data, add the new edge to it, and set it back as the lines of the
poly data, the process fails in the subsequent BuildLinks.

The question is now evident: How can I insert and edge in a vtkPolyData?

Petru