```js
let ManualHider = function(rules) {

this.run = function() {
this.list_observer = new MutationObserver(this.callback);
this.list_observer.observe(document.querySelector(this.matching_unique_rule.root), {
childList: true
});
}

this.task = Promise.resolve()
this.callback = ()=> {
this.task = this.task.finally(async()=>{
await new Promise(r => setTimeout(r, 3000));
// do sth asynchronously
})
}
};

new ManualHider(rules).run();
```