    writer = vtkXMLUnstructuredGridWriter()
    writer.SetInputData(ugrid)
    writer.SetFileName('polyhedron.vtu')
    writer.SetDataModeToAscii()
    writer.Update()

--
    gw = vtkXMLUnstructuredGridWriter()
    gw.SetFileName(fn)
    gw.SetInputData(G)
    gw.Write()
    print('---> ')

