anchor.diff:       ?_assertEqual({ok, [<<"obj01">>]}, fold_keys(Folder, [], [{index, Bucket, {range, <<"$key">>, <<"obj00">>, <<"obj01">>}}], State1)),
        ?_assertEqual({ok, lists:sort(Keys)}, fold_keys(Folder, [], [{index, Bucket, {range, <<"$key">>, <<"obj0">>, <<"obj31">>}}], State1)),
        ?_assertEqual({ok, []}, fold_keys(Folder, [], [{index, Bucket, {range, <<"$key">>, <<"obj31">>, <<"obj32">>}}], State1)),
  +     ?_assertEqual({ok, [<<"obj01">>]}, fold_keys(Folder, [], [{index, Bucket, {eq, <<"$key">>, <<"obj01">>}}], State1)),
        ?_assertEqual(ok, stop(State1))
       ].
   
  
positive.code: fold_keys_fun(FoldKeysFun, {index, FilterBucket, {range, <<"$key">>, _, _}}) ->
      %% 2I range query on special $key field...
      fold_keys_fun(FoldKeysFun, {bucket, FilterBucket});
  fold_keys_fun(FoldKeysFun, {index, _FilterBucket, _Query}) ->
      fun({{Bucket, _FilterField, _FilterTerm, Key}, _}, Acc) ->
              FoldKeysFun(Bucket, Key, Acc);
  
negative.code:     fun() ->
              key_range_folder(Folder, Acc0, DataRef, {Bucket, Min}, {Bucket, Min, Max})
      end;
  get_index_folder(Folder, Acc0, {index, Bucket, {eq, <<"$key">>, Val}}, DataRef, IndexRef) ->
      get_index_folder(Folder, Acc0, {index, Bucket, {range, <<"$key">>, Val, Val}}, DataRef, IndexRef);
  get_index_folder(Folder, Acc0, {index, Bucket, {eq, Field, Term}}, _, IndexRef) ->
      fun() ->
              index_range_folder(Folder, Acc0, IndexRef, {Bucket, Field, Term, undefined}, {Bucket, Field, Term, Term})
  