anchor.diff:  v2 extend  
   1 cells member x  x off
   1 cells member y  y off
  -: print print .attr x .attr y ;
  +: print  print  .attr x  .attr y ;
   extended
   { increased -> 2 cells }
   
  
positive.code: window extend
  rect instance inner  300 inner dim x !  11 inner pos x ! 
  rect instance outer  400 outer dim y !  12 outer pos x ! 
  : print inner print  outer print ;
  extended
  { increased -> 26 cells }
  
  
negative.code: { increased -> 5 cells } 
  v3  extend
  1 cells member z  z off
  : print  print  .attr z ;
  extended
  { increased -> 1 cells }
  
  