private Node<K, V> prevNodeGroup( Node<K, V> node ) {
        if( node.mGroupLeft != null ) {
            node = node.mGroupLeft;

            while( node.mGroupRight != null ) { node = node.mGroupRight; }

        } else {
            while( node.mGroupParent != null && node.mGroupParent.mGroupLeft == node ) { node = node.mGroupParent; }

            node = node.mGroupParent;
        }

        return node;
    }