default ToDoubleBiFunction<V, V> ofIndex(final int index) {
		return (u, v) -> distance(u, v, index);
	}
Return a function which calculates the distance of two vector elements at
a given {@code index}.

@param index the vector index
@return a function which calculates the distance of two vector elements
jenetics.ext/src/main/java/io/jenetics/ext/moea/ElementDistance.java