[vtkusers] Implementing geometrical objects linked together

[nelson -]

hi all,
  i'm a very very very newbie of VTk. I want to use it in a 3D
geometry educational software. I'm able to display simple objects on
the scene (i'm programming in python). Now my question is: how can i
implement using VTK linked objects (like the middle point of 2 points
in the space? I can do it implementing my own observer pattern like
framework or there is something already esisting in VTK i can use?

The problem is to be able to have the middle point changing when i
move the other points...

any suggestion?

thanks,
  nelson