anchor.diff:              ?SCALARIS_RECV({qread_done, _ReqId, _NextFastWriteRound, _OldWriteRound, Value},
                               case Value of
                                   ?NON_EXIST_VAL -> {fail, not_found};
  +                                {error, Reason} -> {error, Reason};
                                   _ -> {ok, Value}
                               end
                             )
           end
       end.
  -
  -
   
   -spec write_helper(client_key(), client_value(), prbr:write_filter()) ->
       ok | {fail | error, any()}.
  
positive.code: -spec read(client_key()) -> {ok, client_value()} | {fail, not_found}.
  read(Key) ->
      read_helper(Key, fun ?MODULE:rf_val/1).
  
  %% @doc Write full JSON object to a given key.
  %% Returns:
  
negative.code:         ?SCALARIS_RECV({qread_done, _ReqId, _NextFastWriteRound, _OldWriteRound, Value},
                         case Value of
                             ?NON_EXIST_VAL -> {fail, not_found};
                             {error, Reason} -> {error, Reason};
                             _ -> {ok, Value}
                             end
                        )
  