@Override protected boolean isFrontierEmpty(){
  cleanUpFrontier();
  updateMetrics(frontier.size());
  return frontier.isEmpty();
}

@ Override protected boolean isFrontierEmpty ( ) { cleanUpFrontier ( ) ; updateMetrics ( frontier . size ( ) ) ; return frontier . isEmpty ( ) ; }
pops nodes of already explored states from the head of the frontier and checks whether there are still some nodes left .
