code: public ArrayList<RepairDigramRecord> toList() {
      ArrayList<RepairDigramRecord> res = new ArrayList<RepairDigramRecord>(this.elements.size());
      RepairQueueNode cp = this.head;
      while (null != cp) {
        res.add(cp.payload);
        cp = cp.next;
      }
      return res;
    }
docs: Needed this for debug purpose -- translates the doubly linked list into an array list.
  
  @return an array list (sorted by priority) of elements (live copy).