[vtkusers] number of points in vtkAppendPolyData

[Denis Saussus]

How can I get the following to print the total 
number of points in the appended set?

a = vtk.vtkPolyData()
b = vtk.vtkPolyData()

apd  = vtk.vtkAppendPolyData()
apd.AddInput(a)
apd.AddInput(b)

print apd.GetOutput().GetNumberOfPoints() 

==> this prints '0' instead of a+b ?

I tried doing apd.Update() before but it didn't help.

What am I missing?