anchor.diff:  
   %% 
   -export([zoom_for_pixelsize/3, 
  +         quadtree/3,
            copyout_tile_for/6,
            calc_tiles_enclosure/2,
            pixels_to_tile/2]).
  
positive.code:     math_utils:xy_assert({3, 3}, pixels_to_tile(1000, 1000)),
      math_utils:xy_assert({3, 39}, pixels_to_tile(1000, 10000)).
  
  -endif.
  
negative.code:     pixels_to_tile(Px, Py).
  
  
  %% @doc Converts TMS tile coordinates to Microsoft QuadTree
  -spec(quadtree(TX::integer(), TY::integer(), Zoom::byte()) -> string()).
  quadtree(TX, TY, Zoom) ->
      Ty = trunc(math:pow(2, Zoom) - 1 - TY),
      quadtree(TX, Ty, Zoom, "").
  
  %% ===================================================================
  %% private functions
  %% ===================================================================
  