anchor.diff:      loop
       88 sprite
   ;
  -
  -\ 
  -\ static int riverat(byte y, uint16_t tt)
  -\ {
  -\   switch (y) {
  -\   case 120: return -tt;
  -\   case 104: return tt;
  -\   case 88:  return 5 * tt / 4;
  -\   case 72:  return -tt / 2;
  -\   case 56:  return tt / 2;
  -\   }
  -\ }
   
   : riverat  ( y tt -- )
       swap case
  
positive.code: ;
  
  : game_setup
  \   Control.begin();
    game_start
    level_start
    frog_start
  
negative.code:     0 GD.TagMask
      drawfrog
  
      t to prevt
      t 1+ to t
  
  