X329: Binary Tree Sum Nodes Exercise (Modified)

Write a recursive function that returns the sum of the values for all of the nodes of the binary tree with root root.
Here are methods that you can use on the BinNode objects:
   interface BinNode {
      public int value();
      public void setValue(int v);
      public BinNode left();
      public BinNode right();
      public boolean isLeaf();
   }