anchor.diff:                              [H|T] -> {H, T}
                           end,
   		    %% Remove process Next from the `active` set and run it.
  -		    NewActive = set_remove(Active, Next),
  +		    NewActive = sets:del_element(Next, Active),
   		    ?debug_2("Running process ~s.~n", [Next]),
   		    run(Next),
   		    %% Create new state.
  
positive.code: %% next and must be moved to the blocked set.
  handler(block, Pid, #info{blocked = Blocked} = Info, _Misc, DetailsFlag) ->
      Lid = lid:from_pid(Pid),
      NewBlocked = set_add(Blocked, Lid),
      ?debug_1("Process ~s blocks.~n", [Lid]),
      case DetailsFlag of
  	true -> replay_logger:log({block, Lid});
  
negative.code:             %% Create the first LID and register it with FirstPid.
              lid:new(FirstPid, noparent),
              %% The initial `active` and `blocked` sets are empty.
              Active = sets:new(),
              Blocked = sets:new(),
              %% Create initial state.
              State = state:init(),
  	    %% TODO: Not especially nice (maybe refactor into driver?).
  