anchor.diff:    {NewLastWakeupTree, NextWakeupTree} =
       case WakeupTree of
         [] -> {[], []};
  -      [{_, _, NWT}|Rest] -> {Rest, NWT}
  +      [#backtrack_entry{wakeup_tree = NWT}|Rest] -> {Rest, NWT}
       end,
     NewLastDone = [Event|Done],
     NewSchedulingBound =
  
positive.code: %% DATA STRUCTURES
  %% =============================================================================
  
  -type event_tree() :: [{event(), integer(), event_tree()}].
  
  -record(trace_state, {
            actors           = []         :: [pid() | {channel(), queue()}],
  
negative.code:         false ->
            case insert_wakeup(Deeper, NewNotDep, Exploring) of
              skip -> skip;
              NewTree ->
                Entry =
                  #backtrack_entry{
                     event = Event,
                     origin = M,
                     wakeup_tree = NewTree},
                [Entry|Rest]
            end
        end
    end.
  