### Question:
NodeCountingReducer implements BiFunction<Node<E>, Collection<Integer>, Integer> { public static <E> NodeCountingReducer<E> get() { return INSTANCE; } private  NodeCountingReducer(); @Override Integer apply(Node<E> node, Collection<Integer> sizeOfSubTrees); static NodeCountingReducer<E> get();  }### Answer:
@Test public void count() { final NodeCountingReducer<Long> reducer = NodeCountingReducer.get(); final int result = TREES .querier() .reduce(tree, reducer); assertEquals(5, result); }