anchor.diff:          NBLOBS 0 do
               i b[] animate
           loop
  +
  +        \ Randomize one blob's velocity
  +        10 randrange 0= if
  +            NBLOBS randrange b[] kick
  +        then
   
           \ Build up a new w*h background image at pad
           pad
  
positive.code:     1 cells var y
      1 cells var dx
      1 cells var dy
      method placement    \ random placement
      method animate      \ compute new position
      method draw         \ draw self
      method brightness   \ return brightness from center point
  
negative.code: 
  :noname
      >r
      rvel r@ dx !
      rvel r> dy !
  ; blob defines kick
  
  :noname
      >r
      centerx 2* rr r@ x !
      centery 2* rr r@ y !
      r> kick
  ; blob defines born
  
  :noname
      dup x @
  