code: function all(type, callback) {
        if (!wm.get(callback)) {
          wm.set(callback, function fn() {
            invoke = false;
            when(type, fn);
            invoke = true;
            resolve(arguments).then(bind.call(broadcast, callback));
          });
          when(type, wm.get(callback));
        }
      }
docs: in case we'd like to react each time to a specific event. In this case a callback is mandatory, and it's also needed to eventually drop.