anchor.diff:  %% @private
   -spec handle_call(term(), {pid(), term()}, #state{}) ->
       {reply, term(), #state{}}.
  +handle_call(enable, _From, #state{enabled=false}=State) ->
  +    replay_debug("enabling tracing.", []),
  +    {reply, ok, State#state{enabled=true}};
  +handle_call(_Message, _From, #state{enabled=false}=State) ->
  +    % replay_debug("ignoring ~p as tracing is disabled.", [Message]),
  +    {reply, ok, State};
   handle_call({trace, Type, Message}, _From, #state{trace=Trace0}=State) ->
       {reply, ok, State#state{trace=Trace0++[{Type, Message}]}};
   handle_call({replay, Type, Message}, From, #state{previous_trace=PreviousTrace0, replay=Replay, shrinking=Shrinking, blocked_processes=BlockedProcesses0}=State) ->
  
positive.code:                     true
              end,
  
              #state{trace=[], previous_trace=Lines, replay=true, shrinking=Shrinking, blocked_processes=[], identifier=undefined}
      end.
  
  %% @private
  
negative.code: 
      case PreviousTrace of 
          [{NextType, NextMessage} | _] ->
              replay_debug("waiting for message ~p: ~p ", [NextType, NextMessage]),
  
              CanDeliver = case {NextType, NextMessage} of 
                  {Type, Message} ->
                      replay_debug("=> YES!", []),
  