anchor.diff:                      ByKey1 = dict:store(Key,Val,ByKey),
                       ByVal1 = delete_key_from_val_idx(Key,OldVal,ByVal),
                       ByVal2 = add_key_to_val_idx(Key,Val,ByVal1),
  -                    {ByKey1,ByVal2,Comp};
  +                    {ByVal2,ByKey1,Comp};
                   _ ->
  -                    T %% maintain monotonicity of Val - it can only increase
  +                    T %% maintain monotonicity of Val - it can only increase (should we really do this here?)
               end
       end.
  +
  +-spec is_empty(tree(_,_)) -> boolean().
   
   is_empty({ByVal,_ByKey,_Comp}) ->
       gb_trees:is_empty(ByVal).
  
positive.code:              dict:erase(Key,ByKey),
               Comp}
      end.
  
  store(Key,Val,T = {ByVal,ByKey,Comp}) ->
      case dict:find(Key,ByKey) of
  
negative.code: %% ===========================================================================================
  %% PRIVATE
  %% ===========================================================================================
  
  -spec insert(Key,Val,tree(Key,Val)) -> tree(Key,Val).
  
  insert(Key,Val,{ByVal,ByKey,Comp}) ->
      ByKey1 = dict:store(Key,Val,ByKey),
  