anchor.diff:  -spec noop_read_filter(term()) -> term().
   noop_read_filter(X) -> X.
   
  --spec noop_write_filter(Old :: term(), WF :: term(), Val :: term()) -> term().
  -noop_write_filter(_, _, X) -> X.
  +-spec noop_write_filter(Old :: term(), WF :: term(), Val :: term()) -> {term(), none}.
  +noop_write_filter(_, _, X) -> {X, none}.
   
   %% initialize: return initial state.
   -spec init(atom() | tuple()) -> state().
  
positive.code:         end,
      _ = case writable(KeyEntry, RoundForWrite) of
              {ok, NewKeyEntry, NextWriteRound} ->
                  NewVal = WriteFilter(entry_val(NewKeyEntry),
                                       PassedToUpdate, Value),
  %%                case kvx =/= _DB of
  %%                    true ->
  
negative.code: %% write_filter(OldLocalDBentry :: custom_data(),
  %%              InfosToUpdateOutdatedEntry :: info_passed_from_read_to_write(),
  %%              ValueForWriteOperation:: Value())
  %% -> {custom_data(), value_returned_to_caller()}
  -type write_filter() :: fun((term(), term(), term()) -> {term(), term()}).
  
  -type state() :: ?PDB:db().
  
  