anchor.diff:  update_bitmap(M = #mfile{offset = Offset}, Pos, Position, Data) ->
       {B, M1 = #mfile{bitmaps = BMPs}} = get_bitmap(Pos, M),
       B1 = set_bitmap(Data, Position - Offset, B),
  -    M1#mfile{bitmaps = maps:put(Pos, B1, BMPs)}.
  +    M1#mfile{bitmaps = btrie:store(<<Pos:32>>, B1, BMPs)}.
   
   read_idx(BaseName) ->
       fold_idx(fun({init, Offset, Size}, undefined) ->
  
positive.code:     set_bitmap(R, I + 1, B1).
  
  get_bitmap(Pos, M = #mfile{bitmaps = BMPs}) ->
      case maps:find(Pos, BMPs) of
          {ok, B} ->
              {B, M};
          error ->
  
negative.code:           index = btrie:new(),
            next = 0,
            otime = undefined,
            bitmaps = btrie:new()
           }).
  -opaque mfile() :: #mfile{}.
  
  