anchor.diff:  make_u_index_key(IndexName, IndexValue) ->
       <<IndexName/binary, ?UINDEX_SEP, IndexValue/binary>>.
   
  +%% TODO(borja): Handle non-binary data
  +make_root_index_key(IndexName) ->
  +    IndexName.
  +
   make_root_index_key(IndexName, IndexValue) ->
       <<IndexName/binary, ?INDEX_SEP, IndexValue/binary>>.
   
  
positive.code: -export([u_index/4,
           read_u_index/3,
           index/4,
           read_index/3]).
  
  -spec u_index(binary(), binary(), binary(), txid()) -> ok.
  
negative.code: 
  -spec index(binary(), binary(), binary(), txid()) -> ok.
  index(IndexName, IndexValue, RefKey, TxId) ->
      MainIndexKey = make_root_index_key(IndexName),
      RootKey = make_root_index_key(IndexName, IndexValue),
      IndexKey = make_index_key(IndexName, IndexValue, RefKey),
      MainUpdate = case claimed_index(MainIndexKey, TxId) of
          false ->
              [{MainIndexKey, ?CLAIMED}];
          true ->
              []
      end,
      Updates = case claimed_index(RootKey, TxId) of
          false ->
              [{RootKey, ?CLAIMED}, {IndexKey, RefKey} | MainUpdate];
  
          true ->
              [{IndexKey, RefKey} | MainUpdate]
      end,
      update_indices(Updates, TxId).
  
  -spec read_index(binary(), txid()) -> {ok, list()} | {error, reason()}.
  read_index(IndexName, TxId) ->
      MainIndexKey = make_root_index_key(IndexName),
      case claimed_index(MainIndexKey, TxId) of
          false ->
              {ok, []};
          true ->
              {ok, Range} = read_index_range(MainIndexKey, TxId),
              Filtered = lists:filter(fun(Key) ->
                  length(binary:split(Key, <<"$">>, [global])) > 2
              end, Range),
              case Filtered of
                  [] ->
                      {ok, []};
                  _ ->
                      pvc:read_keys(Filtered, TxId)
              end
      end.
  
  -spec read_index(binary(), binary(), txid()) -> {ok, list()} | {error, reason()}.
  read_index(IndexName, IndexValue, TxId) ->
  