code: public static <E extends Comparable<E>> Builder<E> naturalOrder() {
      return new Builder<E>(Ordering.natural());
    }
docs: Returns a builder that creates immutable sorted multisets whose elements are ordered by their
  natural ordering. The sorted multisets use {@link Ordering#natural()} as the comparator. This
  method provides more type-safety than {@link #builder}, as it can be called only for classes
  that implement {@link Comparable}.
  
  <p>Note: the type parameter {@code E} extends {@code Comparable<E>} rather than {@code
  Comparable<? super E>} as a workaround for javac <a
  href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6468354">bug 6468354</a>.