guava/src/com/google/common/graph/ElementOrder.java
public static <S extends Comparable<? super S>> ElementOrder<S> natural() {
    return new ElementOrder<S>(Type.SORTED, Ordering.<S>natural());
  }
public static <S extends Comparable<? super S>> ElementOrder<S> natural() {
    return new ElementOrder<S>(Type.SORTED, Ordering.<S>natural());
  }
Returns an instance which specifies that the natural ordering of the elements is guaranteed.