Beginner's Rep Question

I'm playing with frame parts and I cannot work out if I am trying to do something that can't be done or if I am being stupid.

How can I reliably change the properties of a frame part on one window only?

Simple case; in sawfish-client it is simple to get a window and get it's frame:

(setq a (select-window))
(setq b (window-frame a))

It is then simple to get to part of the frame - and change the values:

(nth 2 (nth 3 b))

 -> shows (right-edge . 90)

(rplacd (nth 2 (nth 3 b)) 100)
 -> shows (right-edge . 100)

Now is where it starts to go wrong. Despite repeated calls to 

(refresh-window a)

the change does not appear on screen - rather the change happens suddenly when the window is redrawn, for instance on a resize. Also, when I create a new window (or resize another window) I find that the change has happened to that frame part on all windows.

Am I trying to do something that cannot be done - or am I going about this stupidly?

Many thanks, Richard.

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Sawfish ML