anchor.diff:  %%%%%%%%%%%
   
   %% @doc Get the current maximum concurrency for the given lock type.
  --spec concurrency_limit(bg_lock()) -> bg_concurrency_limit().
  +%%      If the background manager is unavailable, undefined is returned.
  +-spec concurrency_limit(bg_lock()) -> bg_concurrency_limit() | undefined.
   concurrency_limit(Lock) ->
       gen_server:call(?MODULE, {concurrency_limit, Lock}, infinity).
   
   %% @doc same as `set_concurrency_limit(Type, Limit, false)'
  --spec set_concurrency_limit(bg_lock(), bg_concurrency_limit()) -> bg_concurrency_limit().
  +-spec set_concurrency_limit(bg_lock(), bg_concurrency_limit()) -> bg_concurrency_limit() | undefined.
   set_concurrency_limit(Lock, Limit) ->
       set_concurrency_limit(Lock, Limit, false).
   
  
positive.code:                        State).
  
  increment_stat_refills(Token, State) ->
      update_stat_window(Token,
                         fun(Stat) -> Stat#bg_stat_hist{refills=1+Stat#bg_stat_hist.refills} end,
                         default_refill(Token, State),
  
negative.code:     {reply, Infos, State}.
  
  %% Returns empty if the ETS table has not been transferred to us yet.
  do_resource_limit(_Type, _Resource, State) when ?NOT_TRANSFERED(State) ->
      {reply, undefined, state};
  do_resource_limit(_Type, Resource, State) ->
      Info = resource_info(Resource, State),
      Rate = ?resource_limit(Info),
  