Trouble with clipping

[Repenning Alexander <ralex@tigger.Colorado.EDU>]

I'm trying to do some trivial clipping in a window. Although I set the
clip region of the window using clip-rect no clipping occurs:

(setq W (oneof *Window*))

(ask W 
  (clip-rect 0 0 10 10)
  (move-to 1 1)
  (line 100 100)
  (move-to 2 20)
  (prin1 "bla bla bla" W))

Neither the line not the text gets clipped. What's the point I'm 
missing here? Does clipping only work for a  subset of graphic
functions, or is there some means to change the window into a "clippable" 
mode?

  Thanks,  Alex