code: public static void putAt(MutableTreeNode self, int index, MutableTreeNode node) {
          self.insert(node, index);
      }
docs: Allow MutableTreeNode to work with subscript operators.<p>
  <b>WARNING:</b> this operation does not replace the node at the
  specified index, rather it inserts the node at that index, thus
  increasing the size of the treeNode by 1.<p>
  
  @param self  a MutableTreeNode
  @param index an index
  @param node  the node to insert at the given index
  @since 1.6.4