anchor.diff:              _Else -> {Lid, {'receive', ReceiveDetails}}
           end,
       NewLidTrace = queue:in(NewLast, TmpLidTrace),
  -    {TraceTop#trace_state{last = NewLast, lid_trace = NewLidTrace}, Trace};
  -handle_instruction_al(_Transition, _ClockVector, TraceTop, {}, Trace) ->
  -    {TraceTop, Trace}.
  +    TraceTop#trace_state{last = NewLast, lid_trace = NewLidTrace};
  +handle_instruction_al(_Transition, _ClockVector, TraceTop, {}) ->
  +    TraceTop.
   
   check_blocked(TraceTop, ClockVector) ->
       #trace_state{blocked = Blocked, enabled = Enabled, nexts = Nexts} = TraceTop,
  
positive.code:                     AllAddState = add_all_backtracks(Next, LocalAddState),
                      %% ?f_debug("AAS: ~p\n",[AllAddState]),
                      UpdState = update_trace(Selected, ClockVector, Next, AllAddState),
                      %% ?f_debug("US : ~p\n",[UpdState]),
                      NewState = add_some_next_to_backtrack(UpdState),
                      explore(NewState)
              end;
          none ->
  
negative.code: is_next_enabled(_) -> true.
  
  %% Handle instruction is broken in two parts to reuse code in replay.
  handle_instruction(Transition, ClockVector, TraceTop) ->
      Variables = handle_instruction_op(Transition),
      handle_instruction_al(Transition, ClockVector, TraceTop, Variables).
  
  handle_instruction_op({Lid, {Spawn, _Info}}) when Spawn =:= spawn; Spawn =:= spawn_link ->
      ParentLid = Lid,
  