code: public void insertNodeAfter(Node insert, Node after, double split) {
          addChild(insert, children.indexOf(after) + 1, split);
          notifyStateChange();
      }
docs: Inserts a node after (right of or bottom of) a given node by splitting the inserted
  node with the given node.
  @param insert    The node to be inserted
  @param after     The node that the inserted node will be split with.
  @param split The weight