[vtkusers] vtkImagePlaneWidget: Pushing with Middle button

[Flo]

Dear all:

In vtkImagePlaneWidget, how should I do to attribute the "Pushing"  
event to the middle button (i.e. rolling the button up or down would  
push the plane forward or backward) instead of the default such  
behavior attributed to: Cliked middle button + Mouse movement.

I tried the planeWidgetZ2->SetMiddleButtonAction 
(vtkImagePlaneWidget::Pushing); but realize that Pushing was  
protected  !

Thanks.
flo.