code: private void add(Node node, int size, int index, int[] itemset, int support) {
          if (node.children == null) {
              node.children = new Node[size];
          }
  
          int item = order[itemset[index]];
          if (node.children[item] == null) {
              node.children[item] = new Node(itemset[index]);
          }
  
          if (index == 0) {
              node.children[item].support += support;
          } else {
              add(node.children[item], item, index - 1, itemset, support);
          }
      }
docs: Inserts a node into a T-tree.
  @param node the root of subtree.
  @param size the size of the current array in T-tree.
  @param index the index of the last item in the item set, which is also
  used as a level counter.
  @param itemset the given item set.
  @param support the support value associated with the given item set.