anchor.diff:  %% queue or spawned and placed in the first queue. Thus, there are
   %% only elements in one queue at a time, so share an ets table queue
   %% and tag it with a positive length if it contains the first queue, a
  -%% negative length if it contains the second queue. The case -1 is
  -%% handled differently for backwards compatibility reasons.
  +%% negative length if it contains the second queue.
   
   %% ---------------------------------------------------------------------------
   %% # start/3
  
positive.code: %% # handle_call/3
  %% ---------------------------------------------------------------------------
  
  handle_call(T, From, #listener{pending = L} = S)
    when is_list(L) ->
      handle_call(T, From, upgrade(S));
  
  handle_call({{accept, Ref}, Pid}, _, #listener{ref = Ref,
                                                 pending = {N,Q},
                                                 count = K}
                                       = S) ->
      TPid = accept(Ref, Pid, S),
      {reply, {ok, TPid}, downgrade(S#listener{pending = {N-1,Q},
                                               count = K+1})};
  
  handle_call(_, _, State) ->
      {reply, nok, State}.
  
negative.code: %% several accepting transports are started concurrently. Deal with
  %% this by placing the started transports in a new queue of transport
  %% processes waiting for an association.
  
  %% ---------------------------------------------------------------------------
  %% # code_change/3
  