code: public static <N extends MutableBinaryTreeNode<N>> N toLeftAssociativity(N node) {
          checkArgNotNull(node, "node");
          N right = node.right();
          if (right == null) return node;
  
          node.setRight(right.left());
          right.setLeft(node);
          return right;
      }
docs: Performs the following transformation on the given MutableBinaryTreeNode:
  <pre>
  o1                    o2
  / \                   / \
  A   o2     ====>     o1   C
  / \              / \
  B   C            A   B
  </pre>
  
  @param node the node to transform
  @return the new root after the transformation, which is either the right sub node of the original root
  or the original root, if the right sub node is null