code: public function on($event, callable $callback, $priority = 10)
      {
          if (!isset($this->callbacks[$event])) {
              $this->callbacks[$event] = [true, []];
          }
  
          $this->callbacks[$event][0] = false;
          $this->callbacks[$event][1][] = array($priority, $callback);
  
          return $this;
      }
docs: Adds a listener to the end of the listeners array for the specified event.
  
  @param string   $event    The name of the event
  @param callable $callback The callback
  @param int      $priority The lower this value, the earlier an event listener will be emitted.
  
  @return EmitterInterface