### Question:
LongSummingReducer implements BiFunction<Node<Long>, Collection<Long>, Long> { public static LongSummingReducer get() { return INSTANCE; } private  LongSummingReducer(); @Override Long apply(Node<Long> node, Collection<Long> childResults); static LongSummingReducer get();  }### Answer:
@Test public void sum() { final LongSummingReducer reducer = LongSummingReducer.get(); final long result = TREES .querier() .reduce(tree, reducer); assertEquals(27L, result); }