src/main/java/com/workplacesystems/utilsj/collections/TransactionalBidiTreeMap.java
private static <K,V> Node<K,V> getGrandParent(final Node<K,V> node, final int index) {
        return getParent(getParent(node, index), index);
    }
private static <K,V> Node<K,V> getGrandParent(final Node<K,V> node, final int index) {
        return getParent(getParent(node, index), index);
    }
get a node's grandparent. mind you, the node, its parent, or
its grandparent may not exist. no problem

@param node the node (may be null) in question
@param index KEY or VALUE