anchor.diff:  -behaviour(gen_server).
   
   %% API
  --export([start_link/5, stop/1]).
  +-export([start_link/5, stop/1, checkpoint/1]).
   
   %% gen_server callbacks
   -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
  
positive.code:     State2 = #state{i_state=I_State} = roll_log_forward(State),
      Reply = CallbackMod:do_pure_op(Op, I_State),
      {reply, Reply, State2};
  handle_call({stop}, _From, State) ->
      {stop, normal, ok, State};
  handle_call(_Request, _From, State) ->
  
negative.code: stop(Pid) ->
      gen_server:call(Pid, {stop}, ?LONG_TIME).
  
  checkpoint(Pid) ->
      gen_server:call(Pid, {sync_checkpoint}, ?LONG_TIME).
  
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  
  init([PageSize, SequencerPid, Proj, CallbackMod, StreamNum]) ->
  