anchor.diff:  v2 instance pos
   v2 instance dim
   100 dim x !  200 dim y !
  -: print print pos print  dim print ;
  +: print  print  pos print  dim print ;
   extended
   
   { increased -> 10 cells }
  
positive.code: { increased -> 5 cells } 
  v3  extend
  1 cells member z  z off
  : print print .attr z ;
  extended
  { increased -> 1 cells }
  
  
negative.code: 
  window clone window2
  window2 extend
  1 cells member xxx  123 xxx !
  rect instance yyy
  : print  print  yyy print ;
  