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