code: public void add(int index, int end, int[] itemset, int support) {
          root.add(index, end, itemset, support);
      }
docs: Add an item set into the FP-tree. The items in the set is already in the
  descending order of frequency.
  @param index the current item index in the item set.
  @param end the end index of item set to append into the database.
  @param itemset an item set.
  @param support the support/frequency of the item set.