docs: public void addToHead(QueueElement qe) {
  
  		checkElementFree(qe);
  		if (head == null) {
  			head = qe;
  			tail = qe;
  		} else {
  			qe.previous = null;
  			qe.next = head;
  			head.previous = qe;
  			head = qe;
  		}
  		qe.queue = this;
  		numElements++;
  
  	}