@Override
	public int hashCode() {
		return Objects.hash(n, k);
	}
Compares two k-subset elements according its natural order.
{@snippet class="CombinatorialSnippets" region="KSubset.compare"}

@param a the first object to be compared.
@param b the second object to be compared.
@return a negative integer, zero, or a positive integer as the
        first argument is less than, equal to, or greater than the
        second.
@throws IllegalArgumentException if {@code !contains(a) || !contains(b)}
jenetics.incubator/src/main/java/io/jenetics/incubator/combinatorial/KSubset.java