anchor.diff:  
   %% @doc Process evaluates the fitting's argument function, and sends
   %%      output downstream.
  --spec process(term(), boolean(), state()) -> {ok, state()}.
  +-spec process(term(), boolean(), state())
  +         -> {ok | forward_preflist, state()}.
   process(Input, _Last,
           #state{fd=_FittingDetails, phase=Phase, arg=Arg}=State) ->
       ?T(_FittingDetails, [map], {mapping, Input}),
  -    Results = map(Phase, Arg, Input),
  -    ?T(_FittingDetails, [map], {produced, Results}),
  -    send_results(Results, State).
  +    case map(Phase, Arg, Input) of
  +        {ok, Results} ->
  +            ?T(_FittingDetails, [map], {produced, Results}),
  +            send_results(Results, State),
  +            {ok, State};
  +        {forward_preflist, Reason} ->
  +            ?T(_FittingDetails, [map], {forward_preflist, Reason}),
  +            {forward_preflist, State}
  +    end.
           
   %% @doc Evaluate the map function.
  --spec map(map_phase_spec(), term(), term()) -> [term()].
  +-spec map(map_phase_spec(), term(), term())
  +         -> {ok, [term()]} | {forward_preflist, Reason :: term()}.
   map({modfun, Module, Function}, Arg, Input) ->
       %% TODO: keydata
  -    Module:Function(Input, undefined, Arg);
  +    {ok, Module:Function(Input, undefined, Arg)};
   map({qfun, Fun}, Arg, Input) ->
       %% TODO: keydata
  -    Fun(Input, undefined, Arg);
  +    {ok, Fun(Input, undefined, Arg)};
   map({strfun, {Bucket, Key}}, _Arg, _Input) ->
       exit({strfun, {Bucket, Key}});
   map({strfun, Source}, _Arg, _Input) ->
  
positive.code:     exit({jsanon, {Bucket, Key}});
  map({jsfun, Name}, _Arg, _Input) ->
      exit({jsfun, Name});
  map({jsanon, Source}, _Arg, _Input) ->
      exit({jsanon, Source}).
  
  %% @doc Send results to the next fitting.
  -spec send_results([term()], state()) -> {ok, state()}.
  send_results(Results, #state{p=P, fd=FD}=State) ->
      [ riak_pipe_vnode_worker:send_output(R, P, FD) || R <- Results],
      {ok, State}.
  
  %% @doc Unused.
  -spec done(state()) -> ok.
  
negative.code:          done/1,
           validate_arg/1]).
  
  -include_lib("riak_kv_js_pools.hrl").
  
  -include_lib("riak_pipe/include/riak_pipe.hrl").
  -include_lib("riak_pipe/include/riak_pipe_log.hrl").
  
  