### Question:
DoubleSummingReducer implements BiFunction<Node<Double>, Collection<Double>, Double> { public static DoubleSummingReducer get() { return INSTANCE; } private  DoubleSummingReducer(); @Override Double apply(Node<Double> node, Collection<Double> childResults); static DoubleSummingReducer get();  }### Answer:
@Test public void sum() { final DoubleSummingReducer reducer = DoubleSummingReducer.get(); final double result = TREES .querier() .reduce(tree, reducer); assertEquals(28.9, result, 0.02); }