public static Collector<Item, ?, Item> toSum() {
			return Collector.of(
				() -> new double[2],
				(a, b) -> {a[0] += b.size; a[1] += b.value;},
				(a, b) -> {a[0] += b[0]; a[1] += b[1]; return a;},
				r -> new Item(r[0], r[1])
			);
		}
Return a {@link Collector}, which sums the size and value of knapsack
items.

@return a knapsack item sum {@link Collector}
jenetics.example/src/main/java/io/jenetics/example/Knapsack.java