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: v2 extend  
  1 cells member x  x off
  1 cells member y  y off
  : print print .attr x .attr y ;
  extended
  { increased -> 2 cells }
  
  
negative.code: 20 outer pos y !
  30 outer dim x !
  40 outer dim y !
  100 yyy pos x !
  200 yyy pos y !
  300 yyy dim x !
  400 yyy dim y !
  extended
  cr window2 print cr
  
  extended
  