code: 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++;
  
  	}
docs: Add the <code>given</code> QueueElement to the head of this
  <code>Queue</code>.
  
  @param qe
  the <code>QueueElement</code> to add to the head of this Queue
  <p>