code: public ComparableItemListImpl<Item> withComparator(@Nullable Comparator<Item> comparator) {
          return withComparator(comparator, true);
      }
docs: define a comparator which will be used to sort the list "everytime" it is altered
  NOTE this will only sort if you "set" a new list or "add" new items (not if you provide a position for the add function)
  
  @param comparator used to sort the list
  @return this